Use Your Head
After practice, Sensei continued his discussion. Basically, it boiled down to this: you can't expect your teacher to tell you that you're doing something wrong all the time. Sometimes, you're doing something fairly subtle, and Sensei has bigger fish to fry, such as how some of the beginners have their footwork wrong. Or, he may have shown you how to do it once, twice, or maybe even three times, and has (very reasonably) decided he's not going to tell you anymore.
But a big part of it is that, at some point, Sensei has to leave you to your own devices and force you to figure things out on your own. This can lead you down some dead ends, where you do something for years or even months before finally figuring out that it doesn't work. But self-examination is a crucial skill to develop. This process forces you to question everything you're doing, right down to the very basics. All those assumptions you made early on; all those teachings you took at face value without challenge - everything needs to be examined. What am I doing, and why am I doing it? Am I doing it right? Is there a way I could do this more efficiently, faster, with more power? Eventually, you are doing everything consciously again, your technique starts to fall apart, and you feel like a complete beginner. That is a good thing. The alternative is to be stuck in a rut, making no progress, polishing the same old, wrong movements. It is a comfortable place to be, certainly, but it's not why you're there.