Check you really are running the latest for PPC Macs. Adobe's last release of Flash Player for PPC Macs was in November, 2010 with release 10.1.102.64 Subsequent releases are for Intel based Macs only.
To check your installed version of FlashPlayer - http://www.adobe.com/software/flash/about
There's this hack which has Flashplayer tell a web site you are really running a newer version. This gets around sites that don't really need a newer version but just act on the "we know best" basis. If a site really does require a newer version then there's nothing you can do except buy a newer computer.
Flash player 11.1 hack on PowerPC - https://discussions.apple.com/message/16990862