Which food is best to try for a kitten with a sensitive stomach ?

My 9 month old rescue kitten has suffered from loose stools since we brought her home 3 months ago. She is obviously fit and well and gaining weight and the vet says it is dietary. It isnt diarrohea just loose. I have tried iams and whiskas but these both disagree....ive just purchased James wellbeloved to try.....any other suggestions?