What device and Android version are you running? As time passes and additional features are added (or features removed) and the version of YouTube you're running gets more and more out of date there comes a time when you simply need to have a more recent version of the app. If your device is running an old enough version of Android that it's no longer supported for new versions of YouTube you may simply be out of luck for watching through the app on that device and may need to bite the bullet and get something more recent (and not just the next generation of the same line as it suffering the same fate is likely only a year behind).
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