Question for bfeeding moms!

I can pump 1.5 ounces after two hours. I nurse every 2-3hours, even at night. Baby is struggling to gain weight. I’ve been power pumping for a week now and no improvement so far. I’m using a brand new Medela pump and baby has been checked twice for a lip/tongue tie and she’ll all good. I eat oatmeal twice a day and drink lots of water. Any tips please. I’m desperate.