It's hard to say what causes it S., but lots of moms experience a low milk supply. Midwives and herbalists, native women and our elders will all tell you that it is a diet low in milk-producing stimulants that is consistently the biggest problem.
Over the centuries, many remedies have been found for low milk supply from nettles (tea) to rye. Look to Traditional Medicinals at the natural food stores - they have a Mother's Milk tea.
When I was nursing, I used to eat rye bread and drink a little beer daily when my supply was low. The bonus there was the beer relaxed both me and the baby so that my milk could come in easily.
Susan Weed wrote a great book called, "The Wise Woman Herbal for the Childbearing Years". You can still find this book in print at Present Moment in S. Minneapolis or perhaps on the web. It is a bible for new moms who want to breast feed their babies, have healthy pregnancies or raise healthy children.
One thing to make sure... ALWAYS DRINK A GLASS OF WATER every time you nurse. Not replacing the water your body loses is the biggest mistake you can make. Keep a glass of water handy each time you sit down and make sure that you drink every drop. Your milk supply will increase and your baby will be happy.