When my son was born prem I was trying to pump breast milk for him. It was takig a long time for my milk to come in and the volume was small so the nurse in the NICU advised me to take a picture of my son home and to look at it while I was pumping as it would help me.

It has to do with hormones I think, and as nicolejoy said, your body has not really prepared for being a mum when your baby is preemie so sometimes it takes a little longer for the "new mum" hormoes to appear but looking at the picture did help me a lot!