Quoted from PinMonk:If both those check out, make sure you have a credit card's thickness of clearance on between your flipper and the flipper bushing by pulling up on the flipper vertically
This. I bet your flippers are installed with no clearance. Being pulled into the coil at an angle causes friction between the plunger and sleeve and will sap power. They probably worked OK at first because everything was new, but once a little pinball dust was generated it grinds into the sleeve.
What you want is the plunger to pull into the coil perfectly straight. Sometimes a bit more than a CC worth of clearance is needed for this.