My Baby rabbit isn't getting Milk!!!!

(Iowa)

Hi
My rabbit (Holland/lionhead) had a litter of eight (one passed).

One of the eighteen day old babies is fragile and isn't getting milk while nursing. I hold the mother and let the baby nurse, but she doesn't get a round tummy like the others when I do it to them.

She is fragile and though she tumbles around, I'm worried she will die. She nibbles on kale, but she can't live of of that.

What do I do? Any help please?