Are you pumping at different times of the day? That's probably why. From what I've read if you pump in the morning you'll get the most because your prolactin levels are at their highest.
If you are EP'ing it could be like pp said, pumping different times a day will yield varying results, as will pumping at different time intervals (like pumping after 2 hours, then 4, then 2, then 6 because you slept, etc).
If you are nursing and pumping it could be different each time because of how much your baby eats.
If you want to make more milk pump more frequently and/or for longer sessions. Make sure to drink lots of water and feed yourself well.
Re: need to pump up production ...
Are you EP'ing or nursing and pumping?
If you are EP'ing it could be like pp said, pumping different times a day will yield varying results, as will pumping at different time intervals (like pumping after 2 hours, then 4, then 2, then 6 because you slept, etc).
If you are nursing and pumping it could be different each time because of how much your baby eats.
If you want to make more milk pump more frequently and/or for longer sessions. Make sure to drink lots of water and feed yourself well.
-H