Quoted from Wolfmarsh:
There's also no reason for this problem to exist in 2018, stop using power supplies that fluctuate with line voltage
Quoted from SpookyCharlie:
We even use a large capacitor / conditioner board to keep the coils robust at all times... we just can't compensate for all show conditions.
Is what it is. Stop by the shop and play it
Flipper force, like any magnetically derived force, is dependent on current, not voltage. If you can maintain the coil temperature, flipper power will approximately track with voltage. As the flipper coils heat up with constant use, resistance increases, current decreases, and force will decrease.
This problem will exist until somebody begins using closed loop current control on the flippers, the same way you control torque with motors.
You implement closed loop coil current control, and feed the coil rail with a SMPS that can provide the required current at low line voltage, and this lack-of-force problem can theoretically be eliminated. Only true mechanical wear will remain.