I'm guessing you have traditional Southdowns, without any imported bloodlines. Did the lamb get to suckle colostrum from the ewe within the first six hours of birth? The colostrum will give not only give it energy when the brown fat it was born with runs out after the first day, but also antibodies to the disease challenges the ewe has encountered, whether in the natural course of things or via vaccination. Is the ewe fussing too much to stand still long enough for the lamb to suckle, or moving away when it tries to? In either case you may have to hold her steady (one person standing astride her shoulders works well) while another helps the lamb to latch on. Test she has milk in both teats and that it's a cream/yellow colour without lumps - consult your vet if so as she may have mastitis. If the lamb doesn't suckle every few hours you will have to top it up with powdered lamb milk or adopt it on to another ewe. Intensive shepherding in the first six hours and then three days following is vital to the lamb both surviving and thriving.