Do you nurse your baby at times, or are you exclusively pumping? A friend of mine has a similar issue with pumping out little milk but strangely, when she breastfeeds, the flow is quite good.
If you are experiencing low supply both ways, maybe eating milk boosters will help (if you haven't already). The same friend would eat foods with coconut milk for lunch (thai, indian foods) and when she gets back to the office, she'd pump and it works.