Issue: On Who Dunnit, the slot reels are either stuck, move only sometimes, or are just jittering.
Solution: First of all determine if the problem is of mechanical or electrical nature. If you have one working reel, try to swap the driving pcb around between the working and the non working one. If the reel is now working, you have an electrical issue.
If you find your issue to be mechanical - the motors are prone to failure due to dirt. Instead of buying new ones, just clean the old ones - a detailed instruction guide is available here - http://keepingmycool.blogspot.co.at/2013/04/how-to-clean-who-dunnit-slot-reel-motor.html
If you find your problem to be electrical and have ruled out the usual - no voltage, connectors, opto not working... - then there's a good chance of the LM339 being faulty. For whatever reason the LM339's on these boards seem to fail quite commonly.