Switch to soy

My daughter has had problems with constipation for the entire time she's been on formula. She was originally breastfed, and we really didn't have any issues other than I lost my milk. She spits up a lot too. Should I switch to soy? Her father needed soy when he was a baby, too. Literally she goes days without pooping and then blows out a diaper, then it's another couple days. She projectile spits up. I just don't know what else to do because she's already on a gentle formula that we found works better than others, but still isn't perfect.