What Causes Vomiting While On Farex 1 Milk Formula?

Hello. My baby is 4 months now and since his birth he has been taking farex1 milk formula. He is on mixed feed. Since past 15 days, I am noticing that he vomits the entire milk when fed farex. Could it be that farex milk formula does not suit him anymore or is it something else? Please advice. Thanks.