software sources - I am unable to update ubuntu 12.04 LTS. Says check internet connection

I cannot install my updates. I used the following information How do I fix a "Failed to download package files" error? but it didn't work.

In the software sources window under the "ubuntu software" tab everything except the source code is checked.
Under the "other software" tab I have Canonical Partners and Canonical Partners (Source code) and that's all.

When i try to update my software it says:

Failed to download repository information

Check your Internet connection.

I have ubuntu 12.04LTS

please help