Registering players for latter stages problem..

I've just been told to register my players for the latter stages of the Champs league but the problem is i can't register or un register any players because for some reason i am unable to elect the player because they have been 'greyed' out, this is rather annoying as the game has taken it upon itself to unregister players who were originally registered e.g. Aaron Ramsey, Jay Simpson and Theo Walcott, and have been replaced by Mannone, Bischoff and Rui Fonte :confused: This also means i'm unable to register Zapater and Marquinhos who i bought during the Jan transfer window.