I had the same problem when I tried to give mine bigger hides. She was using her old one as a hat more or less, so I decided to make some bigger ones and she refused to use them for several weeks. She would poke her head inside of them but wouldn't lay inside for any extended period of time. Eventually she relented and now she seems to love them as much as her old ones.

They are such creatures of habit that she probably just needs more time to adjust to it. And painting it a darker color couldn't hurt either