Follow along with the video below to see how to install our site as a web app on your home screen.
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Have you tried restarting? That fixes a lot of things. Or clearing data? That will rule out some setting in your Youtube app's Settings menu that might be the cause. Or as @Xavier Black suggested, use a web browser and going to https://m.youtube.com . If you experience the same problem that would indicate a possible bandwidth or some kind of video resolution issue, if not that would indicate the app on your phone is probably the problem.
But whatever the case, if you would please provide more details and what you have or haven't tried to diagnose your problem, it would save a lot of repetition and make it easier for more suggestions.