I have the same problem. I shifted after the ritual for the companions, but after that whenever I tried again, nothing would happen. I went on playing through the game and then at the final main quest I randomly shifted while brawling with Shor in Sovngarde. I thought it had something to do with my race (dunmer) so I tried nord and breton with no luck.
I know I'm necro-bumbing this but I would really like some help with this as lycanthropy is really the only thing I haven't experienced in the game. Also, I'm on PS3 so there's no hotkey problems. And my shouts work fine, it's only turning werewolf.
Thanks ahead