Backup Exec V8.0

My backup and been running fine until a couple of days ago, it got to the “loading” stage and then it started to ask me to remove the media. Then it asked me to insert an overwriteable media. I have checked the properties to overwrite, my tapes are set to overwrite. Sometimes it tells me that my media is unrecognizable. I have tried different tapes and also created new media sets, but the same problem. It wont back up, it stays on “loading”.