Feeding

My daughter is just about 5 weeks old. She went to her 1 month check up this week. Doc said she didn't gain enough since her last visit at the beginning of October. She was 8lbs. 6 oz. (she lost 10% of birth weight initially and then got back up close to it after a week of formula, her birth weight was 8lbs 8oz) this week she was at 9lbs 4oz.

Normally she takes 2oz every 2-3 hours, but doc wants her to get up to 4oz. I've been trying since her appointment but most feedings she'll burp after 2 and then it's a hit or miss if she'll finish even another oz. Since her appointment I make her 4oz bottles and let her eat until she's done but she still usually stops at the 2oz mark..

Any advice? Doc told me to push her to every 3 hours, but even then it hasn't seemed to change her fullness.. Do I just have a lighter eater?