Is it possible that there is an almost invisible typo in those names? Think about a capital ‘i’ rather than an ‘l’, or a ’hard space’ (on a Macintosh you can type a ‘hard’, non-breaking space if you hold the option key while typing it)?
That may be correct. I ended up manually selecting all the images that had the particular musician in them, then in one mass action deleted and re-added the keyword (which I clicked on in the "recent keywords" list). I then unselected the group (having color-coded the set in case my idea didn't work), and ran the search again. This time it picked everything up. So my immediate problem is solved.
But one thing I did notice is that, when I had all the images selected, and thus all the keywords of all the images appeared in the keyword tags window, the musician's name appeared twice. In other words, what I had looked like "A, B, C, C, D," when I was expecting "A, B, C, D." I didn't examine the apparent duplication closely to see if there was some subtle difference between them. I just deleted them both and replaced them.
Ok. The problem was user error. As you suspected, I had spelled the name two different ways "Joaquim" as well as "Joachim," and just hadn't picked up on it.