There is one Sutta in the scriptures called the Canki Sutta (it is in those twenty that I edited if you have them or want to see them in the future), in which a young Brahmin student, who was sixteen, was questioning the Buddha about how to reach the truth. The Buddha pointed out various ways of how to reach the truth. And by the end of a fairly long dialogue, the Brahmin goes, "Oh, yes, yes, now I see how to reach the truth." This is one particular Sutta where I can almost see the Buddha giggling a little. And he says to the young Brahmin something like, "But that is not the final arrival at truth." You can almost see the young Brahmin kind of perplexed, he thought he just had it worked out, but this is not the final understanding that he has. So he says, "Well, sir, what is the final arrival at truth?" The Buddha says to the young Brahmin, "You must do exactly as I said over and over and over and over".
OK, so if the D/D doesn't work, within one time or a short time or one year or two years, keep doing it over and over and over. It is a very good technique and it will work if you keep doing it properly.
When you think about the overall practice, you have heard me say this on the first night of the retreat, it is so much like normal everyday life. Learning to swim, learning to be a doctor or heart surgeon, you've got to keep going, you have to keep practicing, practicing. And if you haven't got it yet, you just need more practice.