Clock is not updating

If you have a task killer, check this link before : Task killer

If you have a home replacement like ADW Launcher or LauncherPro please check the following :
ADW has a persistent mode, so uncheck it.

For LauncherPro verify that the scrollable widgets option is checked and to deactivate the "keep in memory" mode.

Please note that those are only examples of what it could be, nowadays there are so much advanced settings in launchers replacement that it could be another setting.

Another reason: you have too much widgets on your home, and the launcher application is running out of memory. Try to remove a few widgets.