First, I wonder did you apply retention changes to all existing media or only to new tapes (if you don’t remember you could create your existing media pool from scratch and re add marked as free media - it might do you more good). That might be the reason why you have some space left. Additionally, the append to tape happens only with the last tape in the media set (check the sequence number in the media pool view).
As Vladimir said, once the last tape in the media set is offline - a new media set is started anyway, so previously used tape is no longer appendable. If you want to export media on a weekly/daily basis – its might be better to use a create media set daily at option plus tape job export functionality.
And here is a link to our almighty Helpcenter: Creating Custom Media Pools