Passing reforms is based on individual opinion mods. Any country that has a neg opinion of you will not support the reform. You can have 100 IA and still have alot of negative relations.
Because you want positive relations, you should start spending IA to improve relations. You can see in the HRE tab who is not supporting your reform. 1 point of IA raises relations by +40, once per ruler reign (*afaik).
Between improve relations, bestow imperial grace, and gifting money, you should be able to adjust any neg relations & get the support you need.
Unless you've run up a ton of AE or something.
edit: when it comes to passing reforms, 50 IA is the same as 100 IA, so when you are about to advance it is usually a good idea to burn off the excess IA anyway to spread out as many + opinion mods as possible.