Is Flash Player (Shockwave Flash) enabled in your browser?
Your plugins are probably broken...
Download the offline installer here: http://fpdownload.macromedia.com/get/flashplayer/current/licensing/mac /install_flash_player_11_osx.dmg
Quit your browser
Go to: Mac HD/Library/Internet Plugins
Trash the Flash Player.plugin file from there
Go to: Mac HD/Library/Application Support/Adobe
Trash the Flash Player folder (the whole folder)
Empty the trash
Mount the DMG and run the installer.
I followed your suggestions, but it didn't change anything.
How can I tell if Flash is enabled? Apparently not from the Adobe help window, since the box that is supposed to show my version of Flash and my operating system is blank.
But how is it that some u-tube videos work and others don't? For example, I can watch U-tube videos about Canon cameras all day long.
Are you using Firefox or Safari?
I use Safari. I have Firefox installed. I just tried Firefox without any additional Flash installations, and it worked on a video that hadn't worked before.
I have posted the problem with Safari Flash installation as a bug with Adobe.