Two things..
There can be a big difference between what you can express and what she gets - expressing needs alet down reflex and it's less likely to come with a pump than a baby. So whne expressing do a bit of relaxation, close your eyes and think milk.
Also
You and DD may be in perfect harmony. What she needs you produce - no spares available.
In that case you have to express l;ittle and often(after every feed?) to kid your body that your DD wants more. Then in 2-3 days your milk supply may well rise a little.