Help with pumping

JulieMarie21

I'm going back to work on Monday and am worried about the amount of milk I've been able to pump.  I have a Medela PISA, it seems to be "sealed" around my breast, and the pump is very strong.  However, it does not seem to be "emptying" me by any rate.  I will get around 2 let downs, but not much milk.  Sometimes when I throw in the towel, my breast still feels full and I can actually squeeze it and get more milk out.  I never had this problem when I was building up my stash, but now that I'm going to need to provide around 16oz/day, I don't think I can pump that much. 

 So far, this is not a supply issue, I think it is a pump issue.  Is there anything I should troubleshoot on the pump?