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
C:\Users\Jon\AppData\Local\Microsoft\
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.