Is she in a small pen, so she can't get away from the lamb? This can help.
They don't have a lot of milk to start with, it's colostrum now and the milk comes in a day or so and increases as the lambs get bigger.
If you're sure she's not letting them feed at all, tie her up or have someone hold her and put the lamb on.
She'll get used to it, first-timers are often a bit shocked by the whole thing!