I have found that pumping is something that improves with practice.
I'm not sure what kind of pump you have, but with my PIS it starts with a "quick" mode that continues for two minutes to stimulate let-down before automatically switching over to a slower speed for really pumping. If I let down sooner I can press a button to speed that process. I assumed if milk was coming out, i shoudl press the button, when that wasn't the case. Now, I almost always let it go for the full two minutes of the stimulation mode, and sometimes I even turn it off and back on again if I still haven't really gotten letdown (I feel it as a tightening sensation in my boobs) by the time it switches over. Sometimes, too, the suction thing-y (not sure what it's called that goes over the nipple) just needs to be repositioned. If I'm busy with something else, like earlier today when I was on the computer, and feel let-down, I just assume it's all going ok. Instead, I looked down about 5 minutes into it and saw I wasn't getting a thing from my left boob. I repositioned the device so that it was really sucking my nipple effectively, and promptly got 3 oz.
For me, when it's working well, it comes very quickly, and is very different from those early sessions when I hadn't really figured it out and barely got 1/2 an ounce. How much I get depends on the time of day and whether I'm doing it as an additional session (which I really only do in the morning if I have a lot of excess milk. If I were to pump right now, in the evening, after just nursing Willa, i'd hardly get a thing) or if I'm doing it to replace a nursing session. If I'm replacing one (and keep in mind I usually go 3 1/2 to 4 hours between nursing or pumping, and Willa eat a lot at one go) I get anywhere from 5-10 oz total, and usually around 8 oz, which happens to be about as much as she often eats at a time. But I think she's a really big guzzler and likes to eat a lot at one session. My friend who has a 7 1/2 month old has never gotten more than 3-4 oz at a time pumping, even when she was at work, but that was plenty since her daughter tended to eat less. If she pumped less during the day than her daughter ate at day care, she'd try and make it up with an extra pumping session at night or in the morning.
That's probably more detail than you needed, but I find it's one of those things that (unlike nursing) isn't terribly natural and can't take a while to find the groove in. So don't get discouraged. It really will get better!
We ended up taking Willa in to the doctor today despite the fact that her fever was better. She just hasn't been herself all day. It looks like she has a virus, but nothing terribly serious. Still, I'm very sad to see my little girl sick for the first time....