With so little information to go on, it is difficult to be sure what is happening. However, the most common reason for losses at this age is weaning enteritis. Sometimes kits have trouble making the transition from mother's milk to solid foods.
What are you feeding them?
Try giving the kits some grass hay and kitchen oatmeal (the large-flake, old-fashioned kind). These foods aid digestion and may help prevent further losses and it's a good thing to do in any case.
Another possibility is coccidiosis. If you lose another, it would be a good idea to open it up and take a look at the internal organs. Coccidiosis shows up in the liver or intestines. I'm not personally familiar with the intestinal form, but liver cocci results in whitish nodules on or just under the surface of the liver.
Hope this helps!
