Usually, your supply will decrease if you are only pumping. You can try supplementing with brewers yeast tablets, be sure to look at pictures of your baby (or your baby) while you pump, and have lots of skin-to-skin contact with your baby.
The amount you can pump is not an indicater of how much milk you produce. Pumps aren't nearly as efficient as babies. Some wowen can't pump at all, most must pump several times a day to get enough for a feeding, and a rare few can pump huge volumes.
The power of the pump matters too. Try www.kellymom.com for more info