S.S.
I agree with the encopresis posts. I also would suggest food allergy testing. My son had this and was diagnosed with celiac disease (he has a severe allergy to wheat and gluten). He can't take Miralax due to allergies as well but he takes benefiber and MOM daily as well as eliminating wheat and gluten. It has made a world of difference for him. I'm about to get my daughter tested for it too, although her case isn't nearly as severe.
Good luck! It's a long process but once you get some answers and treatment, it gets much easier!