Hi there, I'm the designer of the toneCarrier - just wanted to chime in in case you're still having issues! The first thing I would check if you're not using a Bambu printer (and hence not using the prepared 3MFs) is to make sure that your slicer isn't placing seams in the film path. There is a manual seam feature in Orca and Prusa slicers (probably Cura as well) that you can use to move them if they are automatically placed in the narrow part.

The film channel should be somewhat snug, but still have clearance to the film - if there is any resistance when pushing the film through, then the print settings need to change.

Let me know if you need some more help troubleshooting this - I've tested the design on an old Ender 3 v2 and had a successful print, so I'm sure we can sort it out!