Dirty chicken bums

Hi all- I am sure this has been asked… I am a fairly new chicken owner and am struggling with some of my flock having dirty butts. I have added roosty’s flock armor but it has not helped at all. It doesn’t smell yeasty and isn’t yellow so I don’t think it’s vent fleet. We currently have 15 chickens and 1 rooster, of them I’d say probably 6 have dirty butts. Is best practice to soak and then deworm? Or something else? It’s starting to warm up where we live and I don’t want them to experience fly strike.

I have fenbendazole that I can add to their water. I do have questions about that… how do you ensure they are consuming the proper dosage each day?

Notes about coop/living area:
We have pine shavings in the coop, I do the deep litter method and they have sand in their run with free access to a large outdoor area daily. I add new bedding weekly and do my best to rake poop from run at least 2-3 times a month—they spend 98% of their time in outside pen, it doesn’t get too dirty. They are fed Kalmbach All Flock pellets with daily treats of black soldier fly larvae/scratch grains and kitchen scraps (items vary).

Pic of an angry lady getting a bath for tax