Quoted from SkyKing2301:Right flipper only (since only right flipper activates lane change) ... take a look at the flipper coil & mechanism under the playfield, IIRC there's a switch on there which drives the lane change. I had this same issue pop up once, and turns out the diode on the switch had come loose ... once I reconnected the diode the lane change functioned normally.
Do you have a picture that shows the "switch" and the associated "diode"?