NT Backups with BackupExec 7.2

May be the wrong section to post this, but I have a client with NT 4 and a WangDat 0 4mm tape drive. They do a full backup on Sunday and incrementals on weekdays. The incrementals work fine, but lately the full weekly fails. Sometimes says device I-O error and sometimes says to put in recognizable media. We’ve used two new tapes, cleaned the drive. Perplexed because the incrementals work fine and the full ones fail. Any ideas? Thanks.