Quoting lolajessup:" Because a pump doesn't stimulate milk production as efficiently. Put a bottle up to the other breast when u nurse and catch that stuff!! Haha"
this is what I do with the first feeding in the morning. LO has been sleeping 6-7 hrs and when he wakes up i'm pretty engorged. I've been catching 1-1.5oz just from what leaks out when he is nursing on the other side!
And ditto on other posts that pumping isn't the same as a baby. Maybe if you pumped right out of the shower (or expressed in the shower), or massage your breast before you pump to loosen things up.