This should be pretty easy.

Does the flipper enable relay pull in when the machine boots? (it is the relay near the top left of the solenoid driver board). If it does, look carefully at the contact INSIDE the relay, these frizzle and are a common source of flipper problems.

The circuit is pretty simple:

B+ is ALWAYS applied to one side of the flipper coils so you can measure this with reference to ground (+48V or around that - not critical, as long as it is there)

Check the flipper enable relay pulls in and that the contacts inside the relay are sound

Check the connectors and wiring as shown in the part of the wiring diagram below.

Play a game....