Why is pumping difficult when breasts are hard?

Toni79

First thing in the morning when DD only feeds off one side, I'll pump the other side when she is done.  By that time I'm pretty full and hard.  It seems like I have to pump forever to get a good flow going.  I'll get a let down after a few minutes, but after pumping for 20 mins I'll only have about 1 oz.  Its softer, but I know there is more in there.  Once soft I'm able to message a little and finally get another let down with some real flow.  Then it's another 15-20 minutes to get the remaining 2 oz.  So i'm pumping for close to 35-40 mins to get 3 oz out of a full hard breast!

PS - I should mention this is my sie that generally produces less anyways. 

But why is pumping difficult when breasts are hard?