There are parts of the
coin path you cannot see even with a flashlight.
Coin acceptors jam for a few reasons: foreign matter, damaged
coins, mis-alignment, and general filth. If you have power to the unit, take it out and clean it thoroughly(see attached tech sheet). 90ish% of the time that gets them working. If they still don't work they could need repair. If they are less than 15 years old the cost is $50 per unit. Older than that, toss them.