That can happen when the filament coil develops a knot. It's definitely a showstopper.she came in to find the printer spool seemed to not be letting out material, like it got a kink in it?
The video mentioned here suggests a cure:
viewtopic.php?f=3&t=1724&p=6803&hilit=spool#p6803
I'm not sure that you can find a knot before it snarls the filament drive, particularly if it's buried down inside the reel, but ... after it happens, you can fix it.
First, fix those pulley setscrews!