Not producing enough at night...

I am 4 weeks PP and my LO gets so fussy after her 8pm feeding. She latches great and eats for a good 20 minutes then all of a sudden she starts screaming when she is still latched. It's her hungry cry. I pumped for the past couple of nights at her 8pm feeding and only got 2 oz. I've had to supplement at night to fill her up. What can I do to increase my supply at night?