Godsmack, seems that one of your player's cover is broken...
That kind of movement is not common and to be honest, it's the first time i heard about someting like that.
If you dont want to return it, maybe some glue could help (im not sure about it), or as err0r tol you before, a skin case will keep both covers tight. If you dont want to get the silicone skin from Meizuscandinavia get it from
here or
here