Sometimes the humidity can get the best of parts of a clone. I'll see mold spores from time to time on leaves that were wet too long. I just pitch that part off, and they root as well as any other.
Did you JUST take the cutting or are they days old?
How much leaf do you have on your clones. Post a pic if possible. It maybe that the lower leaves are using up all the water and none is getting to the top.