Well, when I went back and looked at a couple of problems, I was able to solve them, so far. It was definitely, then, an issue of time. (When I find the solution, I expect, it is completely clear and the missing frame is fully specified, and it's reasonably simple. I.e., "obvious in retrospect," as you wrote.)

I do know, independently, that my "multiprocessing" abilities have declined, and that these would be likely be important to any algorithm for solving these problems. I'm sure I could improve my time with practice.

Thanks for your kind thoughts and for the link. I'll check it out.