Queue members with status 'Invalid'

Hi. I’m running asterisk 11 with freepbx 2.11, and have about 12 agents attending 5 queues, when monitoring them, I find that some of the queue members (when doing a queue show) appear as invalid. When I tell them to logout and re login (with *45). I find they are NOT logged out of those queues and are not recieving any calls from them either.

Any ideas as to why does this happen and how to prevent it or at least fix it?