Girls, we've been feeding them for three months. We only feed them Ivs…

Girls, we've been feeding them for three months. We only feed them Ivs (premium formula 1). They ate normally and everything was fine. We fed every two hours with 90 ml. It worked well. But for two weeks they only drink 60 ml. Of course, I will talk to the pediatrician too. But I would like to know from someone who had such experience.