Constipated toddler

My second born is always uncomfortable to poop. Her poop is super stingy, black in color and hard. She doesn't poop regularly. Sometimes it takes up to 5 days gap for her to poop again. We changed her milk na several times as to pedia's prescription, also gave her pobiotics supplement but still pooping is very hard for her. Any suggestions or same situations po? What did you do to help your child?