I need some advise ASAP!

I am 5 weeks pp and I mostly breast feed my LO on demand. so my milk supply is good. however, everytime I try to pump, all I ever get from both breast is 1 oz. when I pump, the flow will be good for a few mins then it'll just stop. my breast don't feel empty at all after that. I've massaged my breast before I start but I can still only get the same amount. am I doing something wrong? because it feels like I am. i was using a Ameda pump I had gotten from WIC timm my insurance came through with a pump. at first I thought it was the pump, but after trying the new one, it does the same thing. help!!!