Screen sharing is generally reliable technology that works whenever both sides of a connection are configured correctly. When problems emerge, they tend be related to your connection method. I've come up with three main types of connection problems, which I cover in turn below. I then finish with General Troubleshooting, which includes problems that could apply to any of the three connection categories.
Because iChat requires two active parties to tango, or agree to share a screen—it's not passive on one end and active on the other—you should have fewer inexplicable problems with screen sharing via iChat and iChat Theater than you would with other forms of screen sharing. If you are having problems, however, run through the following list to find a solution:
Can you see your buddy in the iChat's AIM Buddy List or Bonjour List? (You can open either list from iChat's Window menu.) If not, make sure they're online.
If you can initiate the process of sharing your screen or sharing your buddy's screen, but the process fails, then consider these fixes:
With iChat, only one side of the connection needs to have a publicly reachable IP address for screen sharing, but if neither side is configured to allow iChat's direct connections, then no connection can be made. See Configure Your Router or Gateway.
If your buddy's computer is running a firewall, that could also be a problem. See Firewall Blocks Access, ahead.
If you have an external monitor attached to a laptop that you are screen sharing from, you may need to remove the monitor and try again.
Do you have a fast enough connection to enable screen sharing or iChat Theater? See Ask the Doctor: What Can iChat Do? (p. 13).