Yep! Here's an example for you:
Corona Lykoi couldn't shift to either her Secui or Lupus forms due to a medical condition. :> I'm not sure what age it started or what it exactly is -- profile just states that it's an unspecified medical malady, unfortunately, and I dooon't think her Wiki expands any on it. :<
I'd assume you'd want something that'd be enough to hinder shifting but not enough to seriously hinder flexibility/mobility, considering Rewdeynetya's past. Oh, unless you wanted to go with some kind of progressive thing...? Maybe it's only really starting to act up and slow her down now. ^^;
I am not a doctor at aaaall but maybe
this resource can help you? One thing I found was
this. It causes sharp pain and other joint problems -- maybe she could experience sharp pain in Lupus form, preventing her from wanting to shift to that form? If not, though, it's totally fine to just say it's an unspecified thing that prevents her from shifting, too.