Make sure your YouTube Premium is updated and is installed on your device. Some video formats may not play properly on outdated versions.

Try using a different media player, such as VLC Media Player, MPC-HC, or PotPlayer, if your current media player is unable to play the downloaded videos. A variety of video formats are supported by these players' built-in functionality.

Playback of videos can occasionally be hampered by temporary and cache files that have accumulated. If you downloaded the movies using a browser extension or other software, clean the temporary files and cache in your web browser as well as your media player.

It's conceivable that the extension is interfering with video playback if you're using a browser extension or add-on to download YouTube Premium videos. Temporarily disable the extension, then try watching the downloaded videos once more.

Make sure your web browser is updated and that it is installed. Bug fixes and enhancements that can fix playback problems are frequently included in updates.

It's conceivable that the files became corrupted during the download process if the downloaded videos won't play. Remove the downloaded files from your computer and attempt downloading them from a trustworthy source once more.

Digital rights management (DRM) prevents some YouTube Premium videos from being played on specific hardware or media players. Make sure the downloaded videos are not DRM-protected as they can have playback restrictions.

An unreliable internet connection can interfere with video playback whether you are downloading or streaming the content. Ensure that your internet connection is steady and dependable.

If none of these fixes work, the problem downloaded YouTube Premium videos not playing can be with the hardware of your device or the downloaded videos themselves. In such circumstances, you might wish to look for additional technical support or ask YouTube Premium for help directly.