I had the same thing happen to me. she was my spouse and not a follower at the time. Seems to be one of the many bugs in Skyrim, I don't think the problem is Ella specific.

check back where you first saw them so for ella it would be the drunken huntsman in whiterun thats where she was when this happened to me i just cant seem to get her to move into my house i spent days building this house (in game days rl was like half an hour) making it perfect for her and she wont move in