Hiding head in grass probably means fed up of flies even if not actually struck. Check ears, horn buds, around the mouth, for any scabs or bloody bits. If found, look whether there seems to be any infection and use terramycin spray if infected, antiseptic otherwise. A fly repellant (mind the eyes) will help.
Depending on where you are and the local mineral deficiencies the lambs may be needing a mineral drench. Up here our grasses are short on cobalt, copper and selenium so the local animal health suppliers stock drenches to top up particularly on those - and a one without extra copper for Texels, which are naturally good at managing on low copper and in fact can be harmed, even killed, by excessive copper in their diets.
Does the sickly ewe lamb walk ok when you get her up? Sometimes they get scald between the toes, become reticent to walk, get bothered by flies, retreat into the long grass...
Sorry if I am hurling too many ideas at you - it's just it could be a host of things and I'm hoping that between us we offer you the right idea(s) of what to look for and how to deal with it!