Are you also getting error messages on your POA? Probably.
When you get a User that is having this problem, take a look through MONITOR in FILE OPEN/LOCK ACTIVITY, at the Users USERxxx.DB file. I bet you will find that it is locked open by another User.
The main fix would be to run Maintenance against the Users Mailbox. The better thing would be to complete a Top-Down rebuild and Maintence on the entire GroupWise system. Especially if you haven't done it in awhile!
Good Luck!