Daughter having trouble going #2?

My 14 month old daughter has always had troubles going #2. She has switched doctors a million times this year and I can never get a straight answer or explaination out of anyone because doctors are always in and out of that office. She drinks VIT D milk ,2 times a day and drinks either water or juice the rest of the day. She eats fruit and veggies. I have tried giving her prune juice and raisin bran .. these haven't worked. I have given her a baby lax and its taken hours to actually work. She grunts and moans and cries when she has to go. This could go on for hours before she even goes just a tiny little bit. Other than taking her to the doctor .. because I'm doing this, what do you think I could do or could possibly be wrong???