What Causes Severe Constipation In A Child?

my 11 years girl didnt stool for 7 days,she was a 26week twin,with cronic constipation,we give her miralax,lots of fruit smoothie(papaya,pineapple,kiwi,mango) also plain water,whole grain bread & cereal.I took her to the pediatricin and she recomeded an enema,she has 1 small light green stool on the day after the enema today on the 9 day she hasnot stool yet...we are so worried please help me