Do you have several Symantec products from different generations installed on your computer? I find the newer Live Update module often fails to work with older Symantec programs (whose own Live Update module is overwritten when installing a new Symantec product).

If the above link doesn't solve your problems, completely uninstall your Symantec programs (including any registry entries, plus a manual delete of leftover Symantec folders after the auto-uninstall), and do a clean install, it works all the time.