Similac only!

Rebecca • Just a mommy enjoying life :)
My baby was allergic to dairy he grew out of it at 4 months old but when I tried to give him enfamil because that is what Wic gives he started throwing it up all over again like his allergy never ended. And I knew his allergy ended he was able to tolerate my leftover breastmilk bags from when he was a newborn. So I was confused. I had him on gentlease for a week he was throwing up spitting up cranky hard stomach he was u comfortable! So I switched to similac sensitive. He is amazing now barely spitting up very rarely does he throw up. He doesn't have gas problems anymore or bad stomach pains either. Has anybody else had that problem with baby only tolerating similac brand? Or vise versa? Any suggestions on why that happened?