Signing player on transfer deadline day Problem

The register players for squad message came up at the same time as comfirming a transfer at the end of transfer deadline day. The player I bought once accepted, didnt appear in the list of players to register. So even though I bought him on deadline day, I couldnt register him in my squad

If your squad is already full then that'll be why - if when registering the squad you leave 1/2 places you can put new signings in those places.