EBF, having trouble pumping?

My lo recently started sleeping "through the night" (a 6hour stretch and then 2hrs usually). I haven't been pumping, but started so that I have breastmilk to mix into his rice/oatmeal cereal and so family can feed him while babysitting. I did pump a little in the beginning, like at 1month old for a week or so, to relieve the engorgement pain but then stopped. Anyways, he's been gaining weight just fine so I'm not too worried but I went to pump and I only get a couple of ounces from both sides? Like. I get just over one ounce from one side. Is it just because my body isn't used to pumping and doesn't let down as much? Or is his new sleep schedule and the long break of not nursing affecting my supply? What should I do to get more from a pumping session?