My daughter is going through this. She will go a week to ten days!

Her problem is she holds it because she is scared it will hurt. Miralax seems to have zero effect but we keep using it. Its a stool softener so maybe its helping soften things up.
I have started giving Phillips Milk of Magnesia every day and now she is finally going every few days. I think once she is on a regular schedule of at least every other day for a couple weeks, I will try to scale back on the MoM. Maybe she will be over her mental road block of it hurting.
My niece had it really bad, too, and her doctor had her on daily miralax and milk of magnesia as well. Also, she has to sit on the potty after every meal for 15 minutes (my sister let her take her ipad) and that helped the most.
Good luck!