Peat has said people sometimes react differently to different milk. He said he's known some people to do better with UHT milk. He has recommended trying different milks to see if there is some that suits you. And the thing about starting small and building up slowly.
[But it still may not work for everyone.]
regular refrigerated whole milk seems to work much better for me than the UHT processed stuff. someone mentioned on here that the more you pasteurize or heat milk the more the lactose turns to lactalose and becomes tougher to digest?