The list of available software updates for the Symbian phones is refreshed every time when entering the SW Update menu if there's more than 24 hours since the previous refreshing.
If there is less than 24 hours since the list was last updated, SW Update application will not contact the server for updating the cache. Wait for the next day for the list to be refreshed again.
Note that you can check for updates also in Nokia Suite. Connect your phone to PC and check for updates in Nokia Suite Software Updates menu.