Is your baby allergic to milk?
If your baby is allergic to milk, switching him or her to soy based formula is not the answer. Being allergic to milk is not the same as being lactose intolerant. Babies with milk allergies should proabably be switched to a hypoallergic or amino-acid based formula. BBC NEWS | Health | Milk allergy in babies ‘missed’.
(Note that the article also alluded that soy based forumulas could cause long-term fertility problems!)