The Media Append Period and Overwrite Period depends on the Media Set to which the Media is associated.
Media Set name = MediaSet1
Append Period = 3 days
Overwrite Protection Period = 1 day
The Media associated to MediaSet1 will Append to the Media for 3 days and after the 3rd day is over it will protect the Media for 1 day, means the Overwrite Protection period starts after the append period is over.
Whenever you run an Lable Media job, quick erase is performed on the Media and the Media is allociated to Scratch Media Set, thats the reason you are able to use this media because is overwritable now after Labling.
I think you much either lower down the Overwrite Protection Period so the media will be ready for overwriting.
Hope this helps !!!