YouTube used to stream videos using the Flash video format and Adobe Flash Player. It wasn't until 2010 that YouTube started a HTML5 opt-in trial so that you'll be able to watch a YouTube video with a HTML5 player.
Therefore, if YouTube fails to play, it probably has something to do with the YouTube Flash player. The solutions varies according to the platform used to playback the YouTube videos as listed out below; on a PC, an Android-based device or iOS devices.
You're highly recommended to install the latest Adobe Flash Player by following these steps:
Method 1: Install YouTube Flash Player
1. Uninstall the older version of YouTube Flash Player and install the latest one. Download Adobe's official uninstaller here. Run the program and select your correct operating system, and then Uninstall Flash.
2. Follow Adobe's instructions for installing the latest Flash Player for YouTube.
3. Once you've downloaded the file, quit your browser first, and then install the upgraded version of Flash player.
4. Finally, open your browser again and visit YouTube. You should now be able to watch your videos successfully.
Right-click on YouTube Flash player to double-check which version is currently installed on your computer.
Method 2: Try YouTube HTML5 Player
Opt in YouTube's HTML5 trial to play videos with YouTube HTML5 player rather than YouTube Flash player. Adding &html5=True to the end of YouTube video URL to use YouTube HTML5 player temporarily. Read more into alternative ways to watch youtube without flash.
Some users have reported that YouTube won't play on their Samsung Android phones when try to play in HQ, while some others said YouTube won't play due to conflicts with Twitter's app. Here's some solutions to fix such an issue.
1. If you can't play any video (not only YouTube) on the phone after some time, just remove the official Twitter app, issued by Twitter, Inc., and re-install it before you give it another try.
2. If it's only the YouTube videos playback problem, update the YouTube app from the Android Market to the latest version.
3. As For HTC Android phones, try the HTC Flash Player if it exists on your Android.
Keep in mind that all iOS devices such as iPad, iPhone or iPod, don't support the Flash videos on website. The pre-installed YouTube app is the only way to watch YouTube videos on iOS devices. The YouTube app works well, but if problem arises, you can try this out. First of all, make sure that your WiFi or 3G network works. Secondly, try to close all the apps that are currently running. Then re-launch YouTube app. Reboot your device if that doesn't take effect.
1. Hold your thumb down on an icon to get them all shaking, then close the open apps.
2. Hold home button plus power button simultaneously for a few seconds to close and restart iOS devices.