A couple of things I can think of. Even though new EOS switch, make sure it's gap is adjusted properly. The other thing is make sure the return spring is not adjusted to tight. You want just enough tension to bring it back. There's a start and hopefully others will chime in.

flipper switches can also cause weak flips.
as can the switch on the game over, or tilt relay, which ever one provides power to the flips.

There are no relays involved in the flipper coil path. All there is are the flipper button switches, and the EOS switches.

So your problem is dirty/pitted flipper button switches, EOS opening too soon, return spring wound too tight, or a problem somewhere in the mechanism itself that is causing the shaft/flipper button to drag.

Sure there are.

The reset relay has a switch between Black (the actual source voltage from the transformer) and BLU-Y-W. Then the game over relay has a switch between BLU-Y-W and Red, and Red goes to the flippers among other things. See sections 12E and 13E on the schematic, along the very top.

Thank you Frobozz! I was wondering where I went wrong???
Love me some Williams EM's!!!

OK, but once solenoid power is on, then those switches would cause everything they power to be weak, correct? There are no other switches in the circuit after that except the button switches and the EOS. Obviously there has to be a switch to kill coil power at game over, but he is not saying that the other coils involved in the game seem weak, just the flippers. And in this case, it's just one flipper which seems to me, anyway, to indicate that it's not a matter of power getting to the flipper button switches and then on to the coils themselves, but rather a problem with that one flipper in particular.

In actual practice, the vast majority of the time, weak flippers are the result of dirty button switches, EOS opening too soon, or something wrong in the rebuild of the flipper (dragging, return spring etc).

You have a point sink...agreed, flip switch pitted, and or everything you stated.

I had an issue with a really dragging flipper on the Hot Line I just did. I rebuilt them like I always did, but the right hand flipper just wouldn't return correctly after the flip. I had the return spring exactly how I took it off. Couldn't figure what was up with that.

Then I looked at how the shaft was inserted into the crank arm. I had left the shaft "up" just a bit, instead of pushing the crank arm all the way onto the shaft, trying to protect against the flipper being too low on the bushing and dragging on the playfield. The game has the metal flippers in it, those have some weight to them. What that caused is a pushing down on the crank, which in turn caused the plunger to drag in the sleeve, and the flipper not to return properly to home position. I pushed the crank all the way onto the shaft and problem gone.

Flippers are generally pretty simple and straightforward to do but they can be touchy at times.

I had weak flippers on my em.. called Steve young pinball recourse to pick his brain and maybe purchase one of his Kitts. He gave me a few tips on adjust .and whammo .
Glass slapping good..
Made sure the flipper Rod was riding center In the coil and made sure the gap was set correct.. .huge difference..

OK, good point, I was responding to the forest without seeing the trees. There are, in fact, relay switches involved in the flippers... but they are incredibly unlikely to be the culprit, even less so in this case where it isn't both flippers that are weak. (The other things made weak by those switches being bad might not be as obvious.)