YouTube videos not loading fully
Kindly uninstall your current copy of Flash and then reinstall new >  Troubleshoot Flash Player | Mac OS

Then quit and relaunch Safari.

From the Safari menu bar click Safari > Empty Cache

Restart your Mac, try a video.

If you continue to have problems, go to ~/Library/Preferences/com.adobe.flashplayer.installmanager

Move the Cache.db file from the com.adobe.flashplayer.installmanager folder to the Trash.

Quit then relaunch Safari, try a video.

Safari extensions can prevent some video from streaming. Go to Safari > Preferences then select select the Extensions tab.

If you have any installed, switch to OFF, quit then relaunch Safari, try again.

If you have ClickToFlash installed in ~/Library/Internet Plug-Ins, move that plug in to the Desktop. Quit then relaunch Safari to test