pump or nurse more. The more milk you remove from the breast the more your body will make.

Are you pumping at all? After she is done nursing pump!
Does she have a tongue or lip tie? That can cause them not to get a lot out in result making your body create less.
There are also teas to help with producing more.
I've heard ice cream and beer really help too. Or things with yeast...

Yeah, how do you know you are not producing enough? Having baby @ the breast will automatically trigger what they need.

Does she latch hun?