This has been a recurring problem in Revit 2008. (have not yet occurred in RAC2009). After displaying correctly for some time, some revision tags don't show the revision numbers anymore. They just go blank. (when you select the tag, it shows a question mark in red). It usually does this whenever we switch from project to sheet revision tracking.
Re-tagging the cloud does not solve the issue. If we have some of these lost tags and some correct tags in a sheet, then the revision schedule in the sheet reports two separate rows for the same revision. (if grouped by revision numbers)
If we go to the properties of the revision cloud, the parameter points to the correct revision.
The only method we have found to bring back the revision numbers in tags is to erase the view from the sheet and add it again. If all the tags in all the views in the sheet are corrected, then the revision schedule corrects itself.
Another workaround is to cut and paste the revision cloud along with the tag in the same place.