Could be a number of things, the slingshot mechanism (plunger, coil sleeve) are worn out and need to be replaced.
Your switches for the slingshots are mis-gapped and need to be adjusted so that a glancing blow closes the switch fully.
I'd start there and see if you don't see improved performance.
If all of that fails and you still want more pop, you can change the wiring on the transformer to "High Tap" which will give you a little more life to all of the solenoids in the game.