HI Karina,
I am the same as you... I have to try a few times to get enough milk to make 120mls... and I only get that because I am on medication (motilium)to increase my milk suppy.
As the others have said, expressing is so different to the baby feeding... apparently not all women are able to express... My ECHN was impressed that I could express 60ml... even with the amount of effort it takes.