Help with baby constipation 😓

Justia
My baby has been constipated for a while and she screams and cries when she has a BM. I've tried all the P foods (prunes, pears, peaches , peas), we tried prunes this morning in her bottle, and water ( which she refuses to drink ) 
Any other suggestions? 
Side note : she is taking Similac advanced, I have a feeling that it is hard to digest because of the iron in there. Should I switch to sensitive ?
ThanksÂ