I think you are doing all you can. Inthis case, it appears that something is wrong. We've had thesame situation (usually during the fall/winter months), when apparentlyhealthy babies cannot maintain body temperature and die. Ican only speculate that it could be a virus, bacterial or congenitalproblem. The babies appear to be well fed, but the tummiesbecome very enlarged, kits become cold and die. Strange thisis that their condition often worsens "after" nursing. Thissyndrome appears to cycle. We didn't have any problems thisyear, but in past years have lost all the babies. Thisnormally doesn't happen during the summer months -- yet is notspecifically temperature related. Bunnies can survive in verycold temps if properly protected. Normal, healthy, well fedkits are able to easily maintain body temperature in the nestbox.
Pam