The FAQs for matched activities specifically says: "What if one of my activities is not matched? We currently do not have a way to force an activity to match other activities."
It would be good to be able to do just that. I have 8 rides, but only 7 "match" despite the 8th being identical (overlaid GPX files) and within 0.05 miles distance. Frustratingly, the missing ride is my fastest, which means I have to remember to compare my best "matched" rides to that one.
I guess this will be easier to do, if Strava holds their matching status in a database, rather than the algorithm re-running the matching each time so forgetting the forcing action