Just the fact the plug-in is 64-bit doesn’t mean, it will be accepted by Cubase. During the Cubase plug-in, Cubase tests the plug-in. Uf a critical error which could lead to a crash appears, Cubase blacklists the plug-in.
So the solution is to make sure the plug-in is up to date. If it is, please get in touch with the plug-in vendort.