Pumping/breastfeeding question

My son will be 2 weeks old this Friday, and he's currently in the nicu because I had him 9 weeks early well right now they have me pumping and bringing him the milk. Well when I first started I was able to get an ounce per boob now it seems like I'm getting less and less each time. And my boobs don't feel full or anything after pumping so I know I got it all. Is that common?! I try to pump every 3 hours sometimes 4 if I'm out but I'm worried its going to dry up 😑 is there anything I can do to increase my milk supply?