pumping breastmilk

My baby is three months tomrrow an I exclusively breast feed but wanna pump some like I did in the beginning but none would come out , any idea why it didn’t wanna come out  ? I know I have a good milk supply I never had any issues with it an I been breastfeeding for past four years , an this is my third baby I breastfeed . She gets enough milk an I can feel my milk I am thinking maybe when I pumped I just was empty from feeding her .. but any idea how to make even more milk ?