what's messing up my pumping?

Sarah
The first couple times I pumped I got 4 ounces no problem in only 10 minutes. But the last few times I struggle for even an ounce. I let down right away but then the flow is slow and stops after a minute or two. I know the flanges are too big – waiting for the right size to come in the mail - So pumping has made my nipples sore – could the expectation of pain be the cause?