How old are they? Do you have any pictures you can post? What are you feeding? Do either of their lower bellies under the vent seem enlarged or fuller than the others’? Do they lay eggs? When I have one that has stuck poop I usually wash them in warm soapy water, and dry them well. If they continue to have it, I will trim excess fluff. The reason for the loose droppings that tend to get stuck is important to figure out. Sometimes they will have a little diarrhea, and then the next droppings will cling on to the messy feathers. Are they drinking more water than usual, getting electrolytes often, or getting a lot of treats or scraps they don’t need? Have you ever wormed them?