It should be resolving by now. It is primarily due to dehydration. Are you having trouble keeping the water cleared of litter, perhaps? Maybe you need to raise the waterer higher, or put something like a paper plate under it to help keep the litter away from the water. Maybe it is to hot in the brooder; overheating will cause dehydration. Your brooder really should be large enough that they can move to a cooler area if they wish. You may need to add another waterer. If you are using pine shavings, maybe you have a batch that is eating more pine dust than the average batch does. You can try a switch to old bath towels, clean rags, hospital blue pads or puppy pads, paper towels -- whatever, for a day or two.
I found it easiest to remove by holding the chick under a small stream of running warm water til the vent was clean. If I didn't get every bit off the feathers, I didn't worry about it. When the butt is dry, put a bit of Vaseline, Neosporin, veggie oil, etc. on it so the next poop will slide off a little better. Thicker stuff like Vaseline worked best for me.
Just some ideas; I hope one or two will help you. Bood luck.