This might be an electrical resistance problem. Thinking it would be a dirty switch/ adjustment issue if it was happening to one or two of the targets. But, if it's happening to all the targets, it might be a circuit issue. Did you look at one of the target relays when you were pressing on its related target. When you push on the target with good force, the target switch will send power to the target relay coil. This will move the relay into the second position and latch it into place, and it will close the playfield insert light switches on the relay. Pressing on the target lightly will send power to the target relay coil, it might might move the relay, but is it sending enough power to the target relay coil to get it to move the relay and latch it? Wondering if this circuit has a Jones plug in it. Are the target relays mounted on the underside of the playfield?