You guys crack me up. The guy wants to know why the "A" sides are OK and the "B" sides are not. You come up with every worn out idea and conspiracy theory since the Warren Commission.
Try it again. Be careful how you pull your slides. You can easily pull the holder just a bit away from the body of the camera. Other than that, Why B and not A? Luck.
Check the "B" side holders for light leaks.
tim in san jose