Not the same problem as a clog or a tangled spool
“Not feeding” covers a specific failure: filament sits still somewhere between the spool and the extruder gear, and nothing you do from the printer’s screen moves it. That is different from a nozzle that clogs after filament has already reached the hotend, and different from the clicking or grinding sound an extruder makes when it is gripping filament but cannot push it through a blockage further down. If you are hearing that sound, start with Fix Extruder Clicking instead. A tangle on the spool itself, where one loop has slipped under another and locked the roll from turning, is also common enough to rule out first; see Filament Tangle for how to spot and clear that specific failure.
What is left after ruling those out, filament that will not move even though the spool turns freely and the hotend is clear, is almost always one of four things: a damaged or swollen filament tip, debris or wear inside the unit’s feed mechanism, a tube that is kinked or the wrong length, or a sensor that cannot see filament that is actually there. Bambu’s AMS, Creality’s CFS, Anycubic’s ACE Pro, Elegoo’s CANVAS hub and Prusa’s MMU3 are different enough mechanically that the part names change, but the diagnostic order holds across all of them.
Start with a hand-fed test, not a screwdriver
Before opening anything, feed a length of filament through the unit by hand, along the same path it would normally travel. Push gently and pay attention to exactly where it stops or where the resistance changes. A unit that accepts filament freely for the first stretch and then binds has a different problem than one that resists the moment you insert it, and this one test rules out entire sections of the machine before you disassemble a part that was never the issue.
If filament will not even register as inserted, check the entry sensor first. Bambu’s AMS and AMS Lite use a hall switch at the feeder that lights an LED white when it detects filament and turns the LED off when the filament is removed. If the LED does not respond at all when you insert filament, the switch itself is the problem, not whatever lies behind it.
A damaged spool or a swollen tip
Spool condition causes more feed failures than anything inside the unit. Creality’s own troubleshooting for the CFS hub lists damaged, warped or oddly sized spools as a direct cause of feed and unload failures, because a spool that cannot turn smoothly puts uneven tension on everything downstream of it. A filament tip that has swollen from moisture, or that ends in a hardened blob from a previous cut, causes a similar problem from the other end: it will not pass cleanly through a tube sized for a round, undamaged strand.
Elegoo’s CANVAS hub, fitted to the Centauri Carbon 2 Combo, is a useful example of how tight this tolerance can be. Its PTFE tube runs a 2mm inner diameter, tighter than some competing systems, and Elegoo’s own troubleshooting notes that any point where the filament has thickened or swollen will catch right there, even if the same tip would have squeezed through a slightly looser tube on another brand’s unit.
Debris and wear inside the feed mechanism
Every one of these systems pulls filament through a narrow mechanical path, and that path collects dust and shavings over months of use, worse with abrasive materials like carbon-fiber blends. Bambu documents this directly for the AMS: filament debris building up inside the internal hub increases resistance until loading or unloading fails outright, and the fix is disassembling the hub for a manual clean rather than anything reachable from the printer’s screen. Creality’s CFS troubleshooting points at the same kind of buildup on the extrusion motor gear and at the limit switch, where debris can stop the switch from triggering even though filament is physically present.
Wear looks different from debris. Pull filament back out of the unit and check the side that normally rides against a drive wheel. Flat or shiny patches there mean the wheel is slipping against the filament instead of gripping it, usually because the wheel is worn or its spring tension has dropped. Bambu calls this out specifically for the AMS’s active extrusion wheel, and unlike a one-time jam, it gets worse over time until the wheel is replaced.
A tube that is kinked, too long, or the wrong diameter
The PTFE tube connecting the unit to the hotend fails often because it is also the part most likely to get disturbed during ordinary use. Too much bend right above the tool head adds resistance that can stop filament from being pulled back during unloading, which is why Bambu sells a small bracket just to keep that bend in check. Creality’s guidance for the CFS is more direct: if you are running a longer tube than you need and seeing repeated feed failures, cut it shorter.
Anycubic’s troubleshooting for the ACE Pro and ACE 2 Pro puts the most common jam point at the buffer mechanism, the small assembly that manages slack in the line between the unit and the toolhead. Anycubic’s own guidance is to try a manual feed through the buffer first, and only remove the cover and Teflon tube if consumables are visibly stuck inside after that test.
How the five systems actually differ
The causes above show up on every brand, but which part you end up chasing depends on the hardware in front of you. If you are still choosing a multi-material system rather than fixing one you already own, Best Multicolor 3D Printer compares how AMS, CFS, ACE Pro and Prusa’s toolchanger differ on cost and complexity.
Bambu Lab’s AMS uses three separate hall switches along the path: one at the feeder, one inside the internal hub, and one at the extruder. Bambu Studio’s Device screen shows a green dot for the extruder switch, which lets you confirm whether filament is actually detected at that point before hunting further upstream. AMS Lite, AMS 2 Pro and AMS HT each have their own troubleshooting reference on Bambu’s wiki, and the general layout carries over even though exact part names shift between models.
Prusa’s MMU3 works on a genuinely different principle. There is no AMS-style hub. A selector mechanism picks one of up to five filaments and pushes it down a shared tube toward the extruder, controlled by an idler arm with its own tension screw. Prusa’s knowledge base and its community forum both point to the same two checks first: re-tensioning that idler screw, and recalibrating the filament sensor. The sensor readout should show solid blocks during a successful load; flickering dashes mean the reading itself is unreliable before you even get to a mechanical cause.
Recurring failure versus a one-off
A jam on a single spool, cleared once and gone, is usually a tangle or a damaged tip and not a sign that anything is wearing out. A failure that keeps happening in the same slot, across different spools, points at a worn or dirty part in that specific channel: a slipping gear, a fouled sensor, or hub debris that a quick shake will not clear. Track which slot or channel keeps failing before swapping parts. On a multi-slot unit, narrowing the problem down to one physical path saves a lot of guesswork later.
None of these systems are more fragile than a plain single-spool feed. They just have more moving parts where something can go wrong, and each one fails in a specific, checkable way rather than randomly.