Temperature issues can cause pasting - - either too high or too low. They probably experienced this just in the purchasing/transport process (too cold), and then there was the added stress of a new situation.
This should calm down in a day or two, but you will want to be sure you:
* feed chick starter and fresh water, available all the time
* keep brooder temp at proper level (as you are doing)
* clean the pasting (as you are doing)
As you may know, picking the dried droppings off can injure the chick. You have probably already developed a method of cleaning them, but I have had good luck with holding their little bums under a gently-running bathroom faucet (warm water). I gently dab with a damp paper towel or rag until the bits soften and fall off. Sometimes this can take a few minutes, but they will fall off. Dry the chick off and bingo, all done.
You should not have to do this more than once or twice. If they are healthy and your brooder conditions are good, pasting is a very temporary occurrence.
Hope that helps. Please keep us posted on how they do!