my game has the exact problem as the OP, the ball lock holder worked intermittently. Recently, in the ball lock test, the arm would not move at all (even after bending the arm up slightly). I touched one of the wires (while still in the test) and it started to arc. Upon close inspection, it turns out the solder was gone (or never there) on that wire, thus causing a poor connection at times.
Once the wires were both re-soldered, the mechanism worked great.
Jesse