What can be understood from the problem, it seems that you want to used different tapes for the backups and after you label and assign them to media sets the jobs does not pick up those tapes.
IF this is the case then you are not confguring the jobs properly.
After the media is labeled, the media is a scratch media and is overwritable, not appendable. If you make that media a part of scratch media, that media take the property of that media set( mean its Overwrite prootected or appendable as per the properties of that media set).
If you want to use a particular media to be used in the backup job, configure the media as scratch media, make the job overwritable (make sure no other media is overwritable), run the bakcup job. It should pick up that media only.