Like shep53 I think pine vit 12 could be the answer. I have had problems with my last born lambs. The first lot of lambs got away OK but those born 2_3weeks later have a problem . All given b12 and the poorly lamb, illthrift, has picked up over 2weeks. Bloods confirm low cobalt and selenium. The lamb that gave me cause for concern, presented with appearing to eat_ that thing sheep do when they're unwell and don't want you to realise. Lethargy, he would be sat cursing watching the others and had an unsteady gait. Hope you find out soon what your problem is.