Why does the Update Manager say 'Check your internet connection'?

Every time Update Manager asks check your internet connection but my internet connection is just fine.

I've tried sudo apt-get update in terminal. Its working fine also.

Please advise me.

I had similar issue on Ubuntu 14.04. To resolve it, when the Software Updater fails go to the settings > Ubuntu Software and change the server Download from to the Main Server as shown in the following image:

It was set on 'United States' and it was working fine, but suddenly this error showed up and as I mentioned changing the server fixed the issue.

I had similar problems at one time and solved it by selecting settings in the update manager and clicking Select Best Server. A series of tests are run to find the most responsive server for your situation.