BackupExec 10d media full

I keep getting backup failures on Symantec’s BackupExec 10d on Windows Server 2003. I use Travan TR7 tapes with 20/40 GB capacity and some of the tapes will give me problems saying that the media is full even though the properties show 16.4 GB available on the tape. Any suggestions or has anyone else run into this problem? Thanks in advance for your replies.