I'm not sure what classifies as a "lot" of WLs, but I definitely agree with what you and others have said. You're right about red flags resulting in Rs. Moreover, a weak app would not result in a lot of IIs, so that's not the problem either. You either have terrible luck, since you keep being good but not quite good enough, or something is repeatedly, consistently happening in your interviews that is not bad enough to kill you but is preventing you from getting over the hump.
Statistically, at just about all schools your post-II odds of receiving an A are somewhere between 25-75%, so if you have more than 4 interviews without receiving an A, something is going on with the interviews. Is there anything you can think of??