How are your Relations with the majority of the HRE states? Each and every one of them gets a vote, not just the Electors.
In a couple of campaigns (Burgundy, Austria, Hungary as an "outsider" Emperor, and Brandenburg), I managed to stay on the good side of the member states by repelling foreign invaders, liberating conquered states from their conquerors, and so on. Relations were usually +100 in most, and higher in quite a few, with no more than a handful still resenting the last time I pounded them, plus high levels of trust for honoring treaties and putting down rebels in other countries. Never had a reform fail.