Not looking at a game or schematic but it was common for Gottlieb to make a 3 bladed Spot target switch.. One blade is common, one is for tripping the relay bank, and one goes to all of the targets that score the same value (50 points).

So, if two of the blades are touching, nothing happens. But, when another target it hit, there is a path through the 50 point switch blade to feed the trip relay coil. Check to make sure all of the triple blades are not touching.

There is another possibility to. The same approach is sometimes done through a make break switch on the trip bank. When the trip relay is un-tripped, a leaf switch routes the target switch to the trip coil. Once tripped, the leaf switch re-routes the target to the 50 point relay. If one of those are touching, it has the same effect described above. In the case of the trip bank, sometimes it is as simple as the bank not being fully seated in the mounting bracket slots (wing nuts). This allows the bank to twist a bit and put the leaf switches slightly out of adjustment.

Is it set to 3 Balls? Most games join sequence targets to make it accomplish-able in 3 ball instead of 5 ball (for which it was really designed).

The most likely cause of both are a make/break leaf switch on the trip bank. There is a switch that, when the relay is un-tripped, routes power to the trip coil (Ace is trip bank position #1). Once tripped, the position #1 switch routes power to the U relay (500 points) but it does this through a make/break switch on trip bank relay position #14. This is the special relay and changes scoring points to scoring replays (sequence complete).