Pumping At Night - Low Milk Supply

Mom32712

My daughter is 6 weeks old and per our pediatricans advice, we have started to introduce a bottle.  My husband gives her a bottle every night before bed.  When my husband is giving her the bottle, I pump.  I am only getting about 1/2 ounce -- pumping for 15-20 minutes.  I know she would be eating a lot more...I have had weighed feedings and she has eaten as much as 5 ounces in one feed!   I am currently block feeding due to forceful let down / oversupply so that is just one side pumping.  But with my oversupply, I would expect to be able to pump more.  Any ideas to why I am only pumping so little??