Can you make sure you are running the latest version. This may be a bug that has been fixed. Go to the Help menu and check for updates to get the latest version.
I just want to say thanks to Mark Miller for all the time he has put in helping me over the past 4/5 days to sort out a problem which in truth I should have picked up on day one. It is very rare to get this level of support anywhere these days so thank you Mark.