What Could Be Causing Constipation?

My four year old daughter has been having dark brown stool that is dry, somewhat sticky and flaky. The consistency is a lot like dry mud. It sticks to her bottom, so I have to wipe her and as I do, it falls off in dry flakes. She has been also dealing with constipation since about October of 2013. I ve visited doctors multiple times, they gave her an enema (which did nothing at all), and about four x-rays-- but I have yet to receive any answers. I ve given her MiraLax and cut constipating food out of her diet, I ve also increased her water intake. I just don t know what it could be. I am desperate for answers. Thanks!