Pumping in addition to nursing the baby is a whole lot different than pumping when you are away from the baby. When you are away, you'll pump what she doesn't get to eat, you know? TOTALLY DIFFERENT than pumping in addition to full-time nursing.
I have been pumping for almost a year now, and I get an average of 4 ounces total per pumping session. DD takes 4 ounce bottles when I am at work. Now that she's a year, she wants a bit more in her last bottle to go to bed for the night (I work nights) so we have been adding a little whole milk.
But, I have always only gotten 4 ounces at that's been just fine for us for a year.
Can you try other horns? Another pump? Did you have flat nipples, by any chance? I did, and it made nursing and/or pumping very painful for a while. Things got better around 3 months, though.