Question Marks In Sheet Set Manager

Once in a while, I get question marks in my Sheet Set Manager, and after some picking she sheets, they go away.

This time, the marks are still hanging on. I have: Project>Discipline>Subset>Sheets
Sometimes when I pick a sheet, all the marks go away, and then, come back; these marks are intermittent.

Is there a utility to repair the DST file or what other solution do I have to fix this annoying issue?

Cheers

Can you post a screen capture of the problem?