Yeah, I don't know the size of your tumbler's drum, but my Lortone tumber (model QT66) has 2 drums, each @ #6 ( listed dimentions are 6.75 dia. x 3.625 deep).
I give sizes so that you might gleen the proper volume of pins for yours. I split the #5 kit between the two drums, so #2.5 each. It doesn't matter how few cases I put in the drums, but I don't fill them more than about 2/3 full. soap and citric acid in small amounts, and I fill with water to level with the top of the cases inside.
I suspect that your drum is MUCH larger, which might just require more pins. You certainly can't have too many pins, so long as the drum is filled so you get a nice, aggressive churn. So try less water and more pins and see if it helps.
Question, is the inside of your drum faceted? If not, you may need to add something inside to help the churning.
I was told that every so often, I should tumble with just the pins and cleaner to remove any deposits/crud that may have dried on the interior of the unit followed by a good scrubbing with a scotchpad especially if the inside is rubberized like the FA tumbler]
After about 10/15 uses, the pins can develop a dirty coating, leading to less effective brass cleaning, so it is helpful to run just the pins for an hour with simple green/water to strip them. I wouldn't be too concerned with the inside of the drum.