BE uses tapes in the sequence outlined in the document below
How Backup Exec searches for overwritable media
This is designed so that your jobs will not fail if there is an overwriteable tape available in the library. It will only use tapes from other media sets if there is no suitable tape in the targeted media set. If you partition your library, then BE will not be able to use a tape from another media set if it is in another partition and your jobs will fail.
You should examine your OPP and make sure that there are overwriteable media BEFORE the job starts. The OPP of a media starts from the end of the job that writes to it. For example, your daily jobs starts at 10 p.m and ends at 11 p.m. If you set your OPP to 1 day, then the tape will only be overwriteable at 11 p.m. on the next day and thus will be not be overwriteable when the daily job starts at 10 p.m. the next day. In this case, the OPP should be set to at most 22 hours.