BX is usually the Last Ball relay, not the First Ball relay (which instead is usually DX). So if BX is energizing and latching, then that's going to prevent the Add Player Unit path from working.

Try unlatching BX instead.

U needs to energize again after points are made, and there's a switch there that needs to close and be making contact there when it does.

And the Last Ball relay needs to be unlatched, and a switch opened on it.

So even though it scored some points before hitting the ball trough switch, have both the U and the Last Ball (BX) been completely checked. Is the U relay energizing again and that switch providing continuity? Is BX unlatched and that switch definitely open?

So here's the path to focus on, highlighted in yellow:
Jet Spin Add Player path

If the Player Unit is resetting OK, then the motor 1A, Z1, and Z2 switches must be working (because that portion is in common with reset and advancing to next ball/player).

For now, let's assume that U, BX and P all checked out, and aren't suspects.

So that leaves the switch at motor position 2C, and the normally closed switch on G as the remaining suspects. This path has to complete to get the first step of the Player Unit, and then the next three come via the switches on the Player Unit and Coin Unit. But since it's not stepping at all, motor 2C or G are the ones to check.

I'd guess motor 2C.

Success?