Did you adjust the EOS switch on your problem flipper? If it opens too soon your flipper will definitely be weak. Check out Vid's guide for some EOS tips:
https://pinside.com/pinball/forum/topic/vids-guide-to-upgradingrebuilding-flippers
I think wiring will be different on the switch since you have 1 upper flipper, though the coil wiring should be the same (though I don't have a BK so this is speculation). Maybe post pics so we can see the differences.