If tab does not reach the matching site in 10 seconds, it is ignored indefinitely. This should only affect very slow sites.

If you manage to isolate this or other problems, please report on https://github.com/basilevs/reusetab-firefox/issues. If you find a particularly slow site, please report it too.
I may also implement a configurable cut-off delay, if this behavior is a deal-breaker for you.