I had this problem on my ACDC. It turned out to be the coil stop was bad. The stop is magnetically isolated via a brass sleeve in the steel bracket. When the stop gets loose, it tears up the brass and then the stop touches the steel bracket and completes a magnetic circuit so the plunger sticks to the stop. One tell tall clue is there will be lots of shiny brass dust under the flipper mech inside you cabinet. If you see a bunch of this shiny brass dust, the stop is toast!
I have had a few of these coils stops fail - it seems like they are not as good of quality as the old Williams stops - never had one of those fail like this.