Do you have InstantScan running on the drives in question?
I have encountered situations before where, although InstantScan is active and reports no detectable errors, the volume databases stops updating and so as far as Retrospect is concerned no new files have been added files have been added, changed, or deleted on the volume. When a file is added, changed or deleted the volume database should be updated fairly soon afterwards so if the file time stamp of the volume database is not updating then InstantScan is not updating.
The InstantScan volume databases are in the C:\ProgramData\RetroISA\RetroISAScans folder. To reset stop the InstantScan services, delete the volume databases, the restart the InstantScan services.