Tape devices can experience problems with writing.
If tape write errors occur in the logs, clean the tape device with a fresh cleaning tape. Keep in mind that cleaning tapes are designed to be used only a limited number of times and must be changed periodically.
Additionally, for some of the 4mm and 8mm drives, the cleaning tape needs to be used two or three times in a row to fully clean the heads.
Try using a new tape to get better results. If a tape encounters a media error while writing data, the tape is marked full. Subsequent backups cannot be appended to this tape. Thus, if there is a single tape, all subsequent backup queued to this tape device fail until a new tape is inserted.