If you're working from some network that requires you to use a proxy in your browser to connect to the internet (likely an office building), that might be it. I had the same issue and adding the proxy configs to the network settings solved it.
- Go to your preferences (Eclipse -> Preferences on a Mac, or Window -> Preferences on a Windows)
- Then -> General -> expand to view the list underneath -> Select Network Connections (don't expand)
- At the top of the page that appears there is a drop down, select "Manual."
- Then select "HTTP" in the list directly below the drop down (which now should have all it's options checked) and then click the "Edit" button to the right of the list.
- Enter in the proxy url and port you need to connect to the internet in your web browser normally.
- Repeat for "HTTPS."
If you don't know the proxy url and port, talk to your network admin.