Quality is the biggest factor imo. There was one summer where the Prairie Farms brand was completely fine for me, and there was another brand purely A2 that was 95% if I had to put a number on it. I have Great Value lactose free milk as a result. Lactose from Land-o-Lakes half-and-half doesn't bother me at all even having several ounces a day, that being about one cup of whole milk's worth, since cream doesn't have lactose.

My dad's theory, growing up on a farm where none of the dairy bothered him, was the cream being removed from commercial milk, and especially pasteurization, removed important digestive factors. He said when he'd milk the cows he said the milk would come out a golden color, and he could only have dairy without issue if it was that color.