Milk supply insufficient

Raigan
Ok all you mommas out there, I have a 3 weeks old that I breast feed and formula feed. After my milk started coming in it slowly increased every time I pumped to the point where I could almost pump 4 oz at one time (from both breast) but it wouldn't happen every time I pumped. And now when I pump I only get about an oz if that much. 
What can I do to increase my supply again? I don't want to give up on giving my baby breast milk even if I'm only making enough for one breast bottle a day.