I'm having similar problem still with a little Suffolk lamb. It was orphaned, not sure if relevant. I have wormed with everything over the past few months, oramec, vecoxin, Benzimole and finally zolvix which has no resistance. That was a few weeks ago and still no improvement. Moved onto live yogurt, pro rumen, pepto bismol and now at a loss.
Phoned vet Friday but she didn't bother calling back! I give up! Lamb is lively, eating well, adorable but skinny and shitty! Given 2 shots of amoxicillin as fly strike Thursday and skin was peeling off and sore between her legs. Looks better today.