[otrs] Can't close tickets - tehy seem to automatically reopen

Hi List, I'm rather new to OTRS and having a puzzling problem. Every time I close a ticket, about 20 minutes later I get a message that it's been re-opened. I close it again and again it gets re-opened. I'm reasonably sure it's not a co-worker playing a joke on me. The procedure I use to close a ticket is to go to the queue, click "Close" above a ticket, check my close type which in this case is called "Issue Resolved", leave a note and click "submit". Wait a few minutes, and I get the note, Your ticket status has been updated to: "open". All help appreciated. Regards, Jim