Lisa,
Sorry to hear about your bunnies dying.  I ran outside to get advice from my
husband as he hammered nails into a new shed. He is a veterinarian  Firstly,
as was already posted rabbits are "cache" mothers.  It is normal for her to
cover her babies and leave them alone for long periods of time.  That's how
she hides them from preditors and hunts for her food.  As far as why the
bunnies are dying he says he can't help and you probably need to call your
local vet.  The mother or babies could be sick, or the mother could be too
yet young.  Frequently the first litter an animal has is at high risk for
dying.  The mother has not yet learned how to "mother".  His best advice
until you can call your local vet is to leave them alone.  Too much
interference may not help.  I know the orphened wild bunnies that are brought
to his animal hospital are very fragile and among the most difficult to hand
raise.  Good luck!  Let us know what happens.
Jane Bradshaw
Lynchburg, VA