Des Moines, IA
From my exp, sometimes the coil stops fail because the compression washer is on the coil stop side and gets wedged underneath the stop and breaks it loose. Every stern game I get in, new or used I move the compression washer to the other side to help prevent this. A loose coil stop plunger will cause the flipper plunger to stick. Stern manuals show it on the non coil stop side (#7 in the pic) but every new Stern I get has them on the coil stop side.
The sticking flipper button can be caused by the plastic lip that stops the inner button from coming out of the housing. Sometimes those wear down. Try putting a E clip where the plastic lip is or replace the button
