I'd love to know the answer to this one! My lo is about 4.5 months old and weighs about 10 pounds (was 4.3 lbs at birth) and presently she's taking only 15-18 oz. per day TOTAL. Can't get her to take any more no matter what. However, she is on high calorie formula, which I guess sort of makes up for the lack of oz. she's taking and she is growing and gaining weight, so....I try not to worry so much.
But, I've read that babies should take approx. 2-2.5 oz. per pound of body weight, so at 10.4 pounds, if your lo is taking 20 oz. then I think she's doing okay.
If you find something that works though, please let us know!