On Ubuntu I use SMPlayer or SubDownloader, which both reliably retrieve subtitles from opensubtitles.org. SMplayer even does this automatically for the file you are currently viewing.
So far I've tried SolEol, FileBot, SubDownloader (which is a bit behind on versions for Mac OS; seems like the API URL for opensubtitles.org changed) and Undertext. None of them work reliably, some of them not at all.