I had two particular lambs with this a couple of years ago. Their mum just didnt seem to want to clean them up. I took some warm water and a cloth out to the field in a bucket and distracted the ewe with food while I grabbed each lamb, wedged it between my legs and wiped the poo off. After 2 or 3 days the problem disappeared - either the ewe decided to clean them or the poo became less sticky, I don't know which. I haven't bred from this ewe again so haven't had the issue since.