We're about three months in but we weigh little. We gain 500 grams eac…

We're about three months in but we weigh little. We gain 500 grams each month. Since two months, we have mixed feeding. Pediatricians say I produce little milk. What should I do? I want to stop formula and switch completely to breastfeeding. How can I establish breastfeeding? The breast is taking well, I eat well, I don't know why we weigh little.