Emptier breasts make milk faster than fuller ones. If milk is regularly and thoroughly removed from the breast, milk synthesis keeps chugging along unrestricted. Of course this is an oversimplification of a very complex process, but this principle has held steady as new research emerges. Mom's diet, her fluid intake, and other factors are relatively minor players in this drama. If none of those are factors, it's exceedingly rare than a mom won't make plenty of milk.
Rare situations DO exist, however.
There were several recent articles in the Journal of Human Lactation on this topic by Daly and Hartmann, and Hartmann's talk from ILCA 1990 is still available on tape. A good lactation consultant usually can readily fix these problems. I don't hesitate to recommend pumps because I see so many babies with poor suck responses at first. The poor suck leaves milk in the breast, which compromises milk supply, resulting in a hungry disorganized baby and no milk. At least with a good pumping routine, we have plenty of mom's milk to work with while we figure out how to help the baby feed better.