windows server 2008 wbadmin not remove old backup

I have a win 2008 R2
To backup I use
1) an iternal HDD ( with schedule backup) that works fine
and
2) a dell rd1000 external cardridge ( external removable device) for
disaster recovery, with two cartridge, with a wbadmin scipt

On rd1000 since 10 days the old backup are not removed so the device become
full after few backup.
I've checked and the is no whadow copies on it.
I remember that for more than 1 month it seems to worked fine, old backups
was removed and never get full.
No errors on event viewer.
Any suggestion?
Thank you
Alessandro