I know it’s been awhile since this was originally posted a loooong ago, but I have this problem with multiple apps. For instance, 2 apps that will not play the next file (The apps Re FileExplorer and VLC). Another app, Documents by Readdle will play all of them, one right after another. One set of files (Just one) FileExplorer will play them right after another. If it was just one app that had the problem, I’d say the odds are that it’s simply the app itself.