Hopefully this will help:
Go to Settings - iCloud, scroll down to Sign Out, tap it and delete the account from your iPad. If you're asked to remove or keep everything on your device, select "Keep". If not, just continue removing the iCloud account.
Then sign in again with your iCloud account, and select "Merge" when it appears on your screen.
If this doesn't work when you try it at first, sign out of the iCloud ID once more and login again.