It is hard to know why guys fail without being there. In my class, we had two that did not pass and it was not because they could not fly. Without knowing the two guys, it is difficult to say why.
If you are just coming out to class, don't stress too much about the high failure rate. It has been high in the past but has been much more reasonable lately. When you get here, come with a humble attitude and do things exactly as you are told to do them. Remember in Japan, the process is just as important, or even more so, than the results.