You may have your report level set too low, make sure that it reports from the Warning level
If Media is showing as Poor, chances are, it is some kind of problem with your media agent. Everytime you get an 'ioerror' error (generally error numbers starting with '90:'), it will mark the media as poor
What 'Backup Logs' are you looking at? Perhaps the command:
omnirpt -report session_errors -timeframe 24 24 (the 24 24 means just go back 24 hours)
will show errors for all sessions in thsi timeframe, or, to narrow it down
omnirpt -report session_errors -timeframe 24 24 -datalist "backup specification name"
Maybe, this might also work
omnirpt -report single_session -session [session_id] Do not include the [ ] marks
But, if there is no way that you can recover session errors, your patch level needs to be checked (we have seen this problem before, and was fixed with patches), perhaps needs to be brought up to date, and, if that is not successful, and you are on DP 6.2x or above, you may need to open a case with the Response Center