Error - Not appendable (Media full)

I am using Backup Exec 2012, latest updates on Windows server 2008. I am doing nightly backups on a rotated set of 10 tapes (1 per business day). I am not concerned with overwrite protection whatsoever, all I want is for the backups to be written to tape - appended if there is enough space, or if not, to overwrite the old data, which will always be 2 weeks old due to the time it takes for a full rotation (1 fortnight).

However, I am finding that when my backup jobs get a tape that is full or close to full, instead of overwriting the old data my backups are spontaneously failing with the errors:

Described against the tape in the Media area:

• Overwrite Protection Period: Overwriteable
• Append Period: Not appendable (Media full) 

Described in the job log against the failed job:

Switching to overwrite operation on scratch media.

When these errors occur, the job ejects the tape typically in the middle of the night and halts the job. Here are my relevant settings:

Media set properties:

• Overwrite protection period: 1 Hour
• Append period: None

Other: