Is your computer on at 3AM?

Something has stopped notifying you and downloading, have you tried going to the Microsoft site and doing a manual update?

When you do it will install the COA software and check for activation of Windows 7. Yes, I know probably already done, but sometimes when you go to the Microsoft download site for updates, it will check the COA and verify that you have an authorized copy of Windows 7. Then things will work again.

Sometimes anti-virus software will block updates.

Here's the help page, which I'll assume you have already read, but just in case?