Copper and selenium(white muscle disease) deficiency can both cause muscle weakness/floppiness.
I would be asking the vet, as they will likely know what will be short in your area, and can test as necessary.
Bear in mind sheep are very sensitive to copper, and can equally succumb to copper poisoning, so don't just give copper unless likely to be or tested as deficient.
Lambs that go off legs can also be joint ill, though you might have noticed some heat or swelling in the joints if this was the problem.
If they're not improving or getting worse, I would definitely suggest speaking to your vet or having them looked at.
Suzanne