I think it is difficult to pump for some women after innitial first 3 months as our breasts adjust to demand and produce exactly how much our babies need. The best time to pump is early morning, your supply is at its highest then. Or in the middle of the night, if you don't mind :lol:. Usually baby can extract much more from your breast than the best pump.
I also found it hard to pump when I HAD TO. Especially when I had to leave soon, I was so stressed out by that that I had no let-down either.
My advice would be, pump in the morning, make sure you are well-hydrated, relaxed, think of your baby while pumping. Some women are able to pump while baby is latched on another breast, that way you already have a let-down, the problem is to position yourself with a baby and a pump.
HTH,