there is a little gear which can get out from it place in time. Check its position and if its moved - use a glue to hold it at place.

here is an example of wrong position of the gear:

If you can't figure out where is the gear in a printer - look at the bottom right corner of the picture:

If you need a replacement part:

• google tne next text "pinion gear 10T 0.3M bore 1.5mm";
• also spare parts for RC helicopters can be applied, for example MH-BP031015