Do you have access to the wheels where you can position them into a "win" and note if the coin-release solenoid activates or not? maybe something as simple as a tired solenoid, burnt/deformed coil or sleeve?

The fact that it 'sometimes' pays out (or not) shows that a jackbot does activate the coin-release, but not everytime...

However, the fault might also be before the coin release, maybe an intermittent driver transistor for the coin release etc.

MM.