Quoted from Jason_Jehosaphat:
Thanks, Manny, for the link. I will do some reading there.
Thanks, Scotty, for your suggestion regarding the coil itself. I don't have a replacement coil, but I can order one from PinballResource.
Today I will swap out the Fliptronics board for one that comes from another machine whose flippers are working properly. That will help me rule out my board if nothing else.
If possible, try to switch ribbon cables as well.
I had a DM that had trouble with the upper left flipper and the ribbon cable was the culprit.