V.T.
Like others wrote, make sure you drink fluids - water, milk, juice. This happens to me too, esp when I'm in a 'rush' at the office. Sometimes my milk will 'let-down' immediately when I start pumping. Other times, I get a few drops and need to wait ~10-12min before another 'let-down' occurs.
My friend suggested oatmeal which can help with milk production. I've also eaten papaya that my mother-in-law suggested. My first pregnancy I used fennugreek to help my milk production too. If you are exercising, drink alot more fluids, as that will reduce your milk supply.
9 months is excellent!