If its only one flipper, i would Check the EOS switch of the one that is not working, This would be under the playfield and is opened when the flipper is at full stroke. make sure it is closing properly when flipper is at rest , it may be just dirty but has to be closed firmly when flipper is at rest . check the flipper button wiring also may be a lose or broken wire. Connector J1 on the Solenoid driver board is the one you could try removing and reseating, it may be the fault...
