Unable to Screen Share in Messages

I often use the screen sharing ability in Messages to provide remote tech support for family (my brother, in this case). Something changed recently and I am now unable to ask to share his screen. The reverse is also true (he is unable to offer to share his screen with me). The options are grayed out.

On both machines, if we click "Video" all the "enable" options at the bottom are checked. We have done screen sharing in the past, but for some reason it's not working now.

We are both running the latest version of OS X, 10.8.4 and we both have good (18Mbps+) internet speeds. I have both Google Talk and iMessages in the accounts screen in Messages preferences. I just emailed him to see what accounts he has in his Messages accounts.

I was hoping someone here has experience with losing this functionality in the past and knows how to restore it.

I was able to fix this problem. I had to make sure a particular conversation with a user was using Google Talk before screen sharing was available. If it was using iMessage, it would not do screen sharing. Once I clicked new message, typed the user's name, clicked the Google Talk part of their contact card, I could initiate screen sharing.