My Daughter Is 12, Weighs 160 Lbs. Diagnosed With X-ray

My daughter is 12, weighs 160 lbs. Diagnosed with x-ray with impaction all in colon. Dr said psuefo-diahrea. Today we tried the cleanout. 10 caps of miralax with 64 oz of flavored water. She only went 4 times. Last 3 dark diahrea. I called nurse and she said Dr wants me to do it again tomorrow. I fear she has been impacted for a month. Why we switched GI doctors. Even tho impacted, still 2 BM A DAY, constipation and diahrea alternating every few days for a few months now. I don't think miralax ALONE is going to clean her out. She has been on miralax since April, off of it 3 years prior, before that from indant to 9 years old. Does she need dulcolax or an enema to get it all out? I'm very worried. She's special needs , ASD. high functioning.