I had the same problem when I first started with chickens. You have to keep their bottoms clean or they can get clogged and die. I soaked their little bottoms in warm water till it softened and then once I learned I made sure to check every ones bottoms every day. They need some greens in their diets. I take them out now for a little bit every day to forage. They need some grit in their tummies to digest the greens. Make sure they have clean fresh water 24/7. The heat lamps are drying too which makes for pasty bottoms.