[CMAC51]

Heinlein’s one weakness was that he wrote poor endings. His message was in the story more than any author I ever read. How do you bring that to a conclusion?

The more powerful the story the more difficult it was to bring it to a finalizing climax. “And they lived happily ever after.” did not work for his story lines. It was a flaw I was more than willing to accept in his case.

[Pablo64]

I think you're on to something with that. A few times I feel like his endings were satisfactory. I think Starship Troopers ended well, as did Glory Road. Also, a lot of his earlier fiction intended for younger audiences naturally ended well, but then, there wasn't the depth of character you see in his larger, better-known works, where you either identify so much with the main character(s) or, as you said, the underlying message, that any ending was anticlimactic, or just plain weak.