Agree with pp- check pump over well, also may have to play with settings to find which works best (how fast/strong suction you need).
Your milk production is at it's lowest in the afternoon-evening and most in early morning hours.
It is also hard to get a let down with a pump when you first start. Took me quite a while. Kellysmom.com has great tips for relaxing while pumping/visualize baby/look at pics etc.
I have found I pump the most at night- I pump one side while dd nurses the other side which helps with let down too. It may be harder in early days too as it will seem like baby is nursing all the time.
Drinking a large glass of water 15 minutes before you pump helps too. Don't get discouraged or tense up if you aren't getting a lot at first, it takes time to get used to pump!