Did the vet give you a diagnosis? Sounds like joint ill from your description, which can be difficult to treat in that some respond and recover and some don't. We have had a bit of a problem with that this year, which we've put down to a different brand of disinfectant for the pens we tried in the first week, we've probably had 50 lambs (out of about 1800) with it, and have had to put 6 down. The rest are recovering. Synulox was our vet's choice of treatment. I'm really annoyed about it as I'm totally obsessed with hygiene in the lambing shed.