Some dont get a full milk belly until after whelping and after the pups start to nurse. The pups need to be encouraged to nurse right after birth. That will stimulate milk production. Plus she should of been on a high calcium diet for at least the past few weeks before she whelps and continuer until the pups are weaned. She should be fine, but if after she whelps and still has no milk within 24hrs call your vet.
Dont know how much research you have done but this will help you.
http://www.debbiejensen.com/whelping.html