The normal things that cause it are either being too cold, too warm, stress, viral, or bacterial. Sometimes food too. Do you have a thermometer in with them? It's worth checking the temperature under and away from the lamp. I've been using a electric hen and found it better for my chicks the last few years. Also doesn't heat the whole box too much which a lamp can do.
Make sure you clean them up regularly as it can kill them if left to block. Make sure it's warm water and a warm room and they are dried totally. You can use a hairdryer. Then if you put some petroleum jelly on it should stop the poop sticking until the problem stops. You can also add chicken vit and mins to help them. I add it to the water. There other trial is give them scrambled egg. If it stops then you know it's food. I'd definitely check the temp first. I have 2 digital thermometer. One under my heat source the other at the end if the box. Hope this is some help