There may be several reasons that trailers or extras that have been automatically retrieved for your library might not play. You must first, of course, ensure that you have set things up so that trailers and extras are available.
Related Page: Cinema Trailers, Extras, & Related Albums
Turn on Cinema Trailers in the App
If you’re trying to make use of the Cinema Trailers feature, you choose whether that is enabled (or disabled) in each individual Plex app. If you have multiple mobile devices or use Plex Web App in different browsers or locations, you’ll need to enable the feature as appropriate in each Plex app.
Your computer’s system time is used as part of the request made to the online service when you try to play an automatically-retrieved trailer or extra. The computer’s timestamp doesn’t need to be exactly correct, but If it’s too far off then the request will be rejected. Luckily, this is something that is easily fixed by setting your computer to automatically sync its time settings.
Your computer can sync the time using a protocol called NTP. All major operating systems will allow you to do this and is normally accomplished by enabling some sort of “set date and time automatically” or “use internet time server” feature.
It’s possible that other things could also interfere. To investigate other possibilities, your log files are useful. If you need assistance, post some details about the issue you’re having and your log files in our support forums.