This problem bugged me for months. After the upgrade, only some of my gadgets appeared on the screen. If I tried to reinstall or upgrade the ones that did appear, they vanished too. Thanks to Flattery on the microsoft forums for the fix:

You need to delete your settings in your users app section.
For example if your user name is "Jon" then go to
Then delete the folder called "Windows Sidebar"
This will delete your users gadget settings so when you go to use a gadget it will reset the folder and settings.