i gave birth to my dd at 26 week while she is i...

i gave birth to my dd at 26 week while she is in hospital im expresing milk for her. I exress every 3 to 4 hours during the day and 7 hour at night. Every time i express i get more milk out of one side then i do at the other side (is this normal) and i get a differint amount every time (is this normal). I want to get more milk out for her but i dont no how . I use a electric breast pump and drain then so my boobs are dry. I get out about between 45-60mls ot of one side and between 50-100mls out of the other what should is normal. It takes about 20 minutes to 40 minutes to get these amounts what should it be. Please help i realy want to breast feed her but i have to wait til she gets out in june to do that.I dont want to lose my milk.