A monk asked Koyo Seijo, "Daitsu Chisho Buddha sat in zazen for ten kalpas and could not attain Buddhahood. He did not become a Buddha. How could this be?" Seijo said, "Your question is quite self-explanatory." The monk asked, "He meditated so long; why could he not attain Buddhahood?" Seijo said, "Because he did not become a Buddha."
Meditating does not equal becoming a Buddha (i.e., being enlightened). It's merely a tool to control and discipline the mind. It should help you get there (and there are some Zen practitioners who would tell you it's the only way to get there), but I doubt anyone would say meditating guarantees it. Most simply, Seijo's explanation means that if he did not become a Buddha then he did not become a Buddha. Period. Sitting in zazen for a million years isn't going to make a difference. Being enlightened occurs when you achieve the right mindstate, both in zazen and in your life. If you fail to do this, then it doesn't matter what else you do.
I had read this koan and meditated on it years ago - well before I bought my copy of Mumonkan - and I'd come up with this solution. However, when I read it this time, I felt like my understanding of the solution was deeper and my understanding of the implications more complex. This suggests a very positive thing to me: that the more I learn while practicing Zen the deeper and more meaningful my responses will become. In a sense, I could return to any of these koans a few years from now and expand my understanding.
I've been keeping all my reactions to the koans in a written Zen journal, as I found it helps cement things in my mind if I write it down. Later, it can be helpful as a sort of a review. This helps me build on what I have learned. Given my discovery about being able to return to koans and get more out of them, I think this journal will be a great way to continue building my understanding.