What Causes Severe Constipation In An Infant?

Hi

My Daughter is 6months and 3 weeks old.
Till six months she was given mix of breast feed and Lactogen 1

We started Lactogen 2, Cerealac (wheat and apple) and some home made fruit puree along with breast feed.
She is always constipated to the point that i have to take out her poop using an ear bud. It is so hard.
We were suggest to give milk of magnesia. I was giving her 2.5 ml daily since last 10 days before bed.
She passes normal stools while she is on Milk of magnesia

Now, I stopped giving her Milk of magnesia and again her poop is super solid (almost like stone) and she cries bitterly while passing it.

Now should i continue giving milk of magnesia? till when. I am sure it would have some side effects. SO i do not want to give it forever.

Should I switch back to lactogen 1 or any other milk powder?
or should I switch to brand other than Cerelac

Please suggest.