Expressing trouble

Laura • Hey I`m laura 19 recently had my baby boy :) been with my fiancé a short time and love him millions !
Hi ladies my baby was born at 28 weeks 3 days and ive been expressing since day 2 . I had a good milk flow the first 3 weeks . Im expressing about 7 - 9 times every 3-4 hours a day however during the day i only seem to get 20ml between both boobs but at night and first thing of the morning i get about 60ml between both. How can i produce more ?