Breast milk supply. The reasons could be improper latch, not nursing or pumping enough to meet supply, certain medications (including estrogen-containing hormonal contraceptives), illness, and dehydration. A rarer reason is sheehan's syndrome (post-partum pituitary gland dysfunction. You could discuss with your OB or pediatrician about fenugreek (otc herb). Some try reglan (prescription) as well.
Nutrition. I agree with the biology and stress answer. Moms nutritional and hydration status as well. Some women are just better producers. Some with bigger breast will produce more milk because they have more breast tissue. More is not the issue. Every mother produces enough for her child. Mother's milk tea is a nice herbal formula that enhances milk supply for each individual. Relax!
Biology and Stress! Most new moms think they are not making "enough" milk, and this can be stressful, even though it is usually not true! The biology of milk-making is complex, but stress can make it harder for your body to work at its best. The main keys to optimizing your best milk production: hydrate yourself, nurse or pump often, and enjoy your baby!