Kristen,
It could be. I had a hard time letting down to a pump. But for only pumping 3-4 weeks without your babies here yet, I think 1.5 oz a pumping session is awesome. You may have to supplement when they get here, but I would expect your supply to increase much more quickly when they get here. And for the first week or two, my daughter only took 1-2 oz a session. I increased quickly, but I already had my daughter when I started pumping (last second placement).
Keep up the good work!
Jennifer