Trouble importing Feature Classes into Feature Datasets

I've had problems importing features into a feature dataset if there is another feature anywhere else in the geodatabase with the same name, even if it's not in the same dataset. I don't remember the error I got and whether it was apparent to me at the time, but I guess if nothing else is working so far that's one more thing you could check...

I know this has been solved already, but I was having the same issue and it turned out that my feature dataset was located in a personal geodatabase that had reached its size limit and that was causing the problem.

...just in case anyone else stumbles upon this and the above isn't the issue...

Try using Feature Class To Feature Class instead of Copy Features. The same coordinate system is not the same thing as identical; also see ArcCatalog says spatial references don't match between feature dataset and feature class with same coordinate system?

Fc-to-Fc is more thorough than copy and will automatically take care of coordinate transformation, if needed, as well as transfer attribute domains and some other goodies.