Nan SensiPro formula changing

So my babe started on nan comfort but was struggling with constipation, we moved her to SensiPro and it was so much better. When I couldn’t find SensiPro in store I bought the new version “sensitive” but with the new formulation it seemed to give her really loose stools and her first bout of nappy rash.
Anybody have any suggestions on whey protein only formulas that are not going to cost us an arm and a leg?