We've been drinking Nutrilon since birth and he started spitting up ph…

We've been drinking Nutrilon since birth and he started spitting up phlegm somewhere in the last month of lactation. Then we switched to Nan Opti Pro and here is the third day again spitting up phlegm. I think we will change the formula again. What do you think about the baby's condition? Should we buy it or not? Who has used it before?