I found the issue was just building on my education. You could teach me concept X until you were blue in the face, and I wouldn't remember a whole lot a few months later. But give me a half-decent education on concept X and then soon after move to something that builds on concept X, and I will retain a far greater understanding of concept X for far longer.

I think it is related to a similar effect I have noticed- when I teach something, I again retain that information much better.