I've seen a couple of games where IA is well above 50 and even then the AI never passes a reform until something happens that causes a different nation to become emperor.
Depending on the Emperor and their personal expansionist policies, it's easy to get into a place where you can temporarily not pass a reform. Too much religious opposition, culture differences (particularly an issue if the Emperor is Italian), over extension, annexing HRE members, too much waring in general... all of this can keep even a player from passing a reform for a bit after hitting 50 IA. If you're seeing them pass after a different nation becomes Emperor, it could be that the previous Emperor just had too many other problems. That doesn't bother me. I haven't seen many games in 1.4 where this goes on indefinitely... of course, most of my games since 1.4 have seen the lack-of-electors issue sprout up before 1550, so there may be something else I am not seeing.