When she goes under the ewe, does her tail waggle? That is usually a sign that a lamb is actually getting some milk.
Does she look as if her sides have collapsed in and her back is hunched up? Then she hasn't fed.
Lambs usually manage to suck so if all seems well then all IS well. If you are really worried, pick up the lamb and feel if her belly is a bit rounded and she is nice and warm, or if she is lightweight and hunched. If you think she hasn't fed, put a clean little finger in her mouth and see if she sucks ie that she has a good sucking reflex.
Don't assume that something is wrong - almost always it's all alright
Ah - in the hills and woollyval have posted while I was typing - how are you both so quick
so two different opinions.
It will to some extent depend on which breed of sheep you have - what are yours JMB?