I have always had very weak pop bumpers on my Charlies Angles.

From memory the Gottlieb pop bumper rods on their rings were about 1/8" longer than other manufacturers. When people mistakenly replace the ring/rods with shorter versions the consequence is weak pop bumper action because the ball doesn't go under the ring deep enough when the switch activates.
If you have another game, compare where the ball touches the pop bumper ring.