Poopy butt/vent gleet

[ColtonR]

So I been keeping chickens about three years. Two different flocks one plymouth rocks and the other japanese bantams. My question is why do my chickens always get poopy butt/ vent gleet??? For the plymouth rocks my setup is a 4x8 coop with a 20x50 run. 8x12 covered area with trees providing shade late afternoon. I have a hanging 5 gallon feeder with lid that I refill every 5 to 6 days with either nature wise or kalmbach feed depending on what my local tsc has. I have two hanging 5 gallon waterers that I refill every 3 to 4 days scrubbing the nipples and cups with a brush. Water always appears super clean. I Also recently started fermenting feed with intent to help their gut. I do a 4 bucket method with lids and vapor locks just as a treat. About 2 table spoons per birds each day. Also have a dust bath with topsoil, sand, wood ash, and DE. Once a week or on hot days I provide water with hydro hen. Also once a week provide poultry cell. Same setup for bantams just smaller scale as I only have a rooster and 3 hens. I'm trying to do the best I can and still butts covered in poop. Is this normal or how do I find the issue? I also cut out all tables scraps about 2 years ago and I free range a couple hours in the afternoon. Which have stopped for severall months at a time and still no change.