Adding a cup of white vinegar to the rinse cycle often helps. If you are using disposable diapers, change them often. Disposable diapers can hold an amazing amount of urine before they leak. Change them every two hours or so. Babies older than six months who eat solid foods may develop allergies to new foods.
Food allergies can cause a diaper rash.
Try only one new food a week. Sometimes a more serious skin infection can develop, and will need further treatment.