the WPC driver / power board has 5 G.I. circuits, you will find J120 & J121 have the same pin outs, (I.E. pin 1 on J120 is the same circuit as pin 1 on J121)

What you need to do is have a look under the playfield and see what colour wires are going to the G.I.s that are not working (I.E. green + green & white or brown + brown & white, and so on)

Next check that those wires are connected to the correct pins on J120 or J121 (if you are not sure let me know what colour the wires are and i will let you know)

Next check that there is power on the corresponding pins to those wires with your multimeter set on ac.

If the wires are falling out of the plug as you have mentioned then you will need to replace the G.I. Plugs and sockets that are on the power driver board and G.I wires . (every wpc machine i have worked on has needed these plugs and sockets replaced)