Coil is usually binary, works or not. Little gray area is there when it has started to melt and is partly internally shorted. Main reasons are usually mechanical, especially too tight gap vertically is bad (=plunger has no space to move freely when entering solenoid and can cause drag). My thumb of rule is that if coil sleeve is hard to get of or put new one inside coil, it's good to replace the coil. That means that coil is "Swollowed" because of heat and is not in factory specs. This also causes that electronics has to "work harder" which affects it's life span.

Fliptronics board should be inspected also, measure all transistors, check all the board traces etc.

EOS has no affect on fliptronics-game. It's just to return flipper to up-position if something pushes flipper back down when hold up. Fliptronics gives 40ms pulse to power-coil to make actual hit, then switches to hold-coil to keep flipper up. WPC works quite fine without EOS.