That's because you bought the coarse grit (12) media -
https://www.harborfreight.com/25-lbs-coarse-grade-walnut-shell-blast-media-92150.html
Buy the fine grit (24) media and you won't have that problem -
https://www.harborfreight.com/25-lbs-fine-grade-walnut-shell-blast-media-92155.html
You probably meant "polishing" additive.
Walnut media "cleans" black carbon fouling by scrubbing sharp cut edges against the brass surface. The sharp cut walnut edges don't really "wear" rather get dirty and using used cut up dryer sheets help collect black carbon fouling to extend the life of media before needing to be replaced.
Many use NuFinish to treat walnut media to help with polishing the brass faster. I usually add 2 capfuls and run the vibratory tumbler while stirring to speed up distribution of polish (Wet polish will clump the media and leave spots on brass surface). I add additional capful of polish like every few tumblings before adding brass.
Keep in mind that you want to run the polish treated media on the "dry" side as wet polish will clump the media and won't polish brass.
If you want mirror shine brass, corn cob media does a better job of polishing (walnut cleans better/quicker) and some mix 50/50 walnut/corn cob for best of both worlds.