There are two deficiencies that might be causing your lamb's problem.
Thiamine deficiency can cause neurological problems.
Selenium deficiency can cause muscular problems.
The mineral blocks you have provided may not supply enough thiamine or selenium.
The deficiencies can be treated by injections. BO SE will provide selenium. It can be injected in the muscle. Injectable thiamine (B-12) should be given in the vein.
Another possible explanation for the neurological symptoms is trauma to the nervous system.
The dexamethasone may have reduced swelling in the nervous system which may be the reason for the slight improvement.
Any improvement, even a slight imrovement, is good.
Ask your veterinarian about additional dexamethasone therapy.