A long birthing can cause some oxygen deprivation and make a lamb seen weak, when its really coping with some nerve damage. Was she covered in darlish yellow birthing fluids? Some other things you can check besides that, cleft palete, a high temperature, (sheep are 102.5 but look for over 103.5), a swollen neck, or anemia (look for blood vessels in her eye whites) Do your lambs get a vitamin AD and E Selenium injection at birth? Have you checked the ewe for adequate milk supplies and is the udder free of wool? Sorry for the kind of shotgun questions and suggestions but its a very general description of a lot of different problems. Gotta narrow it down some.