Lift the playfield and place a business card in the flipper opto switch - or something similar. Close the front door, and with the playfield still up, switch the game on and start a game. Does it still flip the flipper ? Then remove the card - the flipper may energize.
You can also swap the flipper optos - left with right.
The plastic flipper opto interrupters loose their tension over time. You can bend them towards the hall switch to "open" the circuit.
