Make sure that it's dark enough in her cage to make it seem like night. If it's only kind of dim, it'll make her feel like she's in a nestbox! A good way to test the light-blocking properties of the cover is to drape it over your own head and look at a bright light. Some covers that seem heavy let a lot more light through than you'd expect.

I did you trick with all the blankets on her cage over my head, it's black as pitch.