This has been an ongoing problem with the upgrade to DP 8, and there have been several patch bundles issued for both DP 8.0 and 8.1 to try and address this. If you are on DP 8.0, I advise that you install the patch bunlde for DP 8.01, download from:
http://support.openview.hp.com/selfsolve/patches
However, even after doing this, you may not see the missing sessions. See if the sessions are in the Internal Database by running the command:
omnidb -session -type backup
If you have a range of dates, you can filter this by running
omnidb -session -type backup -since yyyy/mm/dd -until yyyy/mm/dd
Just as an example
omnidb -session -type backup -since 2013/11/15 -until 2014/05/01
If you see a session ID there that you are not seeing in the Restore GUI, get eh session ID and run
omnidb -session [session id] -detail Do not include [ ] marks
to see if the session is protected, and to see if it shows something like 'Missing Media'
If the session is not there, or if it shows Missing Media, you will need to import the media back into the IDB. There is a file 'media.log' (since you did not specify the OS-version of the cell manager, I can't be more specific) which includes the media used for each session. It should not be not affected by the Upgrade. Import em media in the order shown in the 'media.log' file