I agree with pp. i used the hospital pump when lo was born and my pis when i got home and there was no change or delay. every lc at the hospital told me the pis was the "cadillac" of pumps! mine is an older model, but you should have a control for increasing suction and one for speed. whenever i've had poor suction, it was one of the following: white membrane wasn't on all the way, tubing came loose from the pump connection or the horns, or the face plate came off slightly. or my shirt was under the shield or i fell asleep and they came off.
if that doesn't work, definitely call medela again.