Keep up with the laxatives until her stool is soft and she goes more regularly. Before thinking about adding fiber, first increase the water intake. When you add fiber always add more water as well.
Make sure you yourself don’t focus too much on the poo problem, approach it with a relaxed attitude.

Keep going with the laxatives until they are nice and soft and she goes ever day or every other day. Then ease back on them. That’s what our gp recommended. My daughter had the same issues and eventually it did get better but it took ages! Good luck.

A high fibre diet will help keep her bowel motion regular, even just adding some bran to her cereal or cook a batch of cookies adding the bran will help.