There are a couple reasons that a piece of content may have returned for a search, but we were unable to highlight the specific places in the text where the search matched. They require understanding a little bit about what we do under the hood. Generally, these are edge-cases that you won't encounter that often and if you do, you can always click, "go to post" to check the post out on the platform itself.
1) It's a link or a hashtag
We treat links and hashtags a little differently to make sure searches work perfectly for both. It these cases, we won't be able to highlight it specifically, but generally it's usually apparent where the link or hashtag appear.
2) We smush all of the text together to keep things speedy.
For any given Facebook post (or tweet or reddit thread), there are a number of different areas with searchable text (The message someone writes, the caption on the link, etc). To keep things quick, we smush all of those pieces together so rather than searching each individually, we just search one big blob. This means that sometimes the end of one section and the beginning of another section might trigger a search hit. For instance, if your search is "U.S. House" and the post has a caption of "Night Clubs are Growing in the U.S." and a link title of "House music is the best.", We may find a match for "U.S. House."
3) We use stemming.
Stemming is the processing matching root words to their extend variants (think "ski" for "skiing"). This makes the results much better for searches by-and-large, but the way its stored makes highlighting impossible. So in the case of a stem match, we can't show a highlight.