My daughter is 6 month old now. There is enough breast milk for her but She is not gaining weight .. why this happens

A. We Not sometimes realise that having sufficient breast milk is not enough it should be healthy also it should be thick enough for the child also maybe you’re having sufficient breastfeed in but it is not thick enough it could b thin like water for not gaining sufficient weight so if you need to you can start with formula feeding as well for the child