AASTRA 6751i logging people out and they can't get back in

Hello all we use AASTRA 6757i phones with a freepbx server. Sometimes usually over the weekend a few users will notice there phone is not signed in and it is asking for their extension and login password. When they attempt to login it says your phone is already logged in here (this is not true). I usually end up having to force the log off by signing into a different phone and then relogging in with their phone works… What gives?