I'm afraid the answer is that some people are simply and completely arrhythmic. I went to a military college and we all had to learn how to march, which I thought was rather simple. the rhythm was a pretty straightforward 1-2 left-right kind of thing, after all, and most people got it. but there were a few select special individuals who could never seem to grasp that concept, let alone something complicated like 4 vs 3 or the like. bottom line: some people are simply rhythmically incompetent, despite all efforts to the contrary. some people may grasp 7 vs 13 immediately, and the rest of us may eventually get polyrrhythms with extensive practice, but a few selected folks will never, ever, ever even come close to getting it.