On my BK2K the upper flipper is fed by a contact on the lower right flipper (it is not a two-stage flipper; you can verify this on your game because there will only be one leaf switch activated by the cabinet button). That is a high-voltage contact on the lower right flipper. I would make sure that switch is filed clean, as well as all the others.
I would also look for any kind of binding in the mechanism. I want the plunger to have as little extraneous motion as possible as the coil pulls in.