The first thing I would do is reseat all the cables on the flipper board and see if that solves the problem. Report back if it doesn't.

That board suffers from faulty fuse clips and cracked solder joints. When the game is off, push down on each fuse - if the clips splay outward, they are bad, and both ends need to be replaced. With the board out of the game, check the solder side of the connectors for tiny cracks around the pins. If you see any, remove the solder and replace with new solder. If I already have the board out, I re-solder the pins anyhow, since you can occasionally miss visual indicators, and it's basically a free repair.

While you are under the hood, I'd go ahead and check the motor control board for the rex and the two relay boards mounted to the bottom of the playfield - I found several cracked solder joints on the rex relay and mine works better than new now.

It might allow some current to pass, but it'll probably be intermittent or weak (or both).