Babies get into a rut.
Their digestive systems go haywire and parents (let’s just say moms) have to move fast and wisely to get them specialized formulas that supply their babies with the nutrition they need to develop and grow in the crucial few minths if life outside the womb.
Once that mom figures out what specific formula is necessary to keep her baby fed, she ignores every other formula while she hunts like an apex predator for that specific formula.
If that specific formula is not available, what else can be said, other than: All hell breaks loose. The babies of those moms are in dire straits and can easily end up in the ER.
When a baby is not keeping down what you are feeding them, it’s not as if you can force the issue with that baby. The baby’s body is in charge, and will buck no argument. Their bodies can end up being the most finicky things. High end Jaguars that need that high octane fuel, or the wheels start falling off.
My youngest is 46, so those days are but a distant (and often hazy, though pleasant) memory, and things have changed a lot since then ... though many changes are not so good.
Cheers! & regards to you both.