There’s a link under the list of matched segments that can often provide an explanation:

Sometimes though, there is no explanation. I recently had 3 segments fail to match during a race, with no explanation for any of them, so I reached out to Support. As soon as they matched those 3 segments to my run, 3 others became unmatched! These, too, lacked an explanation, which leads me to believe there may be a cap to how many segments can be matched to an activity.

This could potentially be based on the total distance of the activity, because I’ve seen very long bike rides with half a million segment matches. Meanwhile, other public activities at my race also failed to match segments that they clearly qualified for.

I would love if a Strava employee could confirm/clarify this.

That'll do it. There was roadwork and a detour shortly before the start of the segment. If you look at the screenshot from Strava, the one-block jog to the east that happens just north of the segment start is the detour. However, I was "back on track" so to speak almost a quarter mile before the segment start. That seems like pretty wide error margin for the start/end of a segment. Especially given that the new "hide" feature for the start and end of activities is half that distance (1/8 mile).

On the other hand, there are a lot more segments matched this time versus last time three years ago. So if the issue description I can see in the UI is wrong, and what you're saying is the real root cause, it would be really helpful to be able to "throw away" segments, and pick relevant ones that didn't match.