I do not have a problem with "generally" not working for you "scientifically". Perhaps I should have used another term.
N - "If one believes probiotics survive stomach acid then they should work effectively in all subjects."
Why would you think that? Does not he pH of the digestive system change and differ from one end to the other? What happens if this is out of Whack? Would not it affect, not only the survivability of a probiotic, but also the probiotic's ability to function as they should in a less than ideal medium; either too alkaline or too acidic? Or with too little or too much oxygen?
There is a lot more at work here (and needed) than the "probiotic" alone in order to achieve a desired result.