This has to do with the 3 mine scrapers.

One was toasted by the minecrawlers, the other, I had to eradicate all those snappers, and I got his report...

That leaves Marcos. I've found him, I've found his scout Grimes, and I've used the rune to talk to Garand. I think I might have messed things up by finding Marcos before even making it to the castle the first time, and listening to his report then, and offering to get him help.

Now after I've talked with Garand and he wants the reports, Grimes tells me where Marcos is, I got to Marcos and he thanks me for the help, and starts walking around. So... I'm left with.... (this isn't verbatim)

Garand: I'm just waiting on that last report....
Grimes: This ore doesn't want to seem to move....
Marcos: Thanks again for getting me help.

....I've talked to each of them repeatedly, hoping to get some other scripts for them to say.... nothing.....

....am I missing something, or did the game just mess up?

Never mind. I'm an idiot.

Just noticed the letter that I picked up from Silvestro's corpse....

I don't know how many times I have done that; beware: there are a lot of letters, etc that do the same sort of thing.