You’ll probably find that your baby drains your breasts effectively, leaving perhaps only 5-10ml of milk to express. Try waiting for 45 minutes to an hour after the feed before you express, to allow your breasts to partly refill, though only if this still gives your breasts enough time to fill before the next feed. You may simply have to wait until your baby is bigger, and has longer stretches between feeds, before you can start expressing on a more regular basis. Or try using a double pump to reduce the time spent expressing, creating more refilling time. When you do express, keep the session as short and effective as you can, so you have maximum refill time.