I've just had my second child - 10 years later! And she has been the opposite in regards to…

I've just had my second child - 10 years later! And she has been the opposite in regards to breastfeeding... She was born tiny and wasn't eating enough. She latched right away (unlike the first one) but wasn't gaining enough weight after her first two weeks in the world. Now she is eating 50/50 breast/formula, but still doesn't seem to be getting enough breast milk at those feedings (don't worry she's gaining well now - because of the formula though) and seems gassy after breast milk as well. More to come - she just woke up!