OS X Yosemite: If you can’t share computer screens
If you’re having problems sharing your screen with another Mac, or viewing a shared screen on your Mac, there are several things to check.
Make sure you turned on screen sharing and set up sharing permissions on the Mac whose screen you want to share. See Share your screen.
If you’re trying to share the screen of another Mac, open Sharing preferences on that Mac and make sure you’re in the list of users who have permission to share its screen.
Make sure neither Mac is in sleep mode. Adjust sleep settings using Energy Saver preferences.
If you’re using Back to My Mac to share screens, make sure it’s turned on on both computers. See Share your screen using Back to My Mac.
Make sure both computers are connected to the same network or, if you’re sharing screens using Back to My Mac, make sure they’re both connected to the Internet.
To check your connection, open the Network pane of System Preferences. The indicator next to your type of network connection should be green.
If there’s a problem with your network connection, use Network Diagnostics to troubleshoot.