My son is 4mths 3wks old now and is eating mush now. Before having mush, he used to have 4 meals with 6 to 7oz every feeding. I changed his nipples for a few times. It is sometimes the water flow of your nipple is too slow and it takes lots of energy for baby to drink milk. If the water flow is too slow, they have to suck very deep and it makes them exhausted. You may consider to introduce mush or changing your nipple.