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.

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).

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.