How to get a tape to be overwriteable and appendable

I am noticing that when I rotate my tapes out for the "Full" backup weekly,  I erase the tapes prior to the backups.  I put the tapes in the correct media group and they are overwriteable BUT it is also "Not applendable".  The tapes have to be appendable to get all the backup sets on these tapes.

What am I doing wrong?  I am not getting a good backup to tape.

Thanks

Cathy