The metal "flipper" that rides across the last coin is probably sticking. Tighten up all the screws, lightly oil the pivot point of that flipper, and if it still does it, order a spring for the flipper. Newer models come with it.
MEP is right, the Lock Lever is the metal part that skims over the last quarter and it is usually replaced, along with the bushing inside of it, when it starts to fail. This is a $2 piece. One option is to send the coin mech to us to refurbish and we will go over everything, and we fix it and return it same day.