Is everything moving freely and not binding or hitting anything it shouldn't? Then check the coil sleeves. Then the coil resistance and are they the right coils for your machine?

If your wall voltage is low you can set your machine to high tap. https://www.pinwiki.com/wiki/index.php?title=EM_Repair#Setting_your_EM_Machine_to_.22High_Tap.22
With today's electrical systems this is rarely necessary.

Here are some things to check in the order I'd look at them

1) Are the flipper switches and end-of-stroke switches clean?
2) If the flipper end-of-stroke switch adjustment is too wide, that can affect flipper strength.
3) Clean the solenoid plungers and coil sleeves
4) See the 5141A and 5141B here http://www.pbresource.com/coilgtb.html

Have you rebuilt the flippers?

This^^^