Trouble with Screen Sharing

I use the iChat screen-sharing service over the internet to help my parents troubleshoot computer issues at long distance.

Lately the trouble is that while my father, within his OS X user account, is able to initiate screen shares with me, my mother, within her OS X user account, can only ask to share my screen -- she does not have the option of inviting me to share her screen.

I had my father create a new OS X user account, and then, within that space, configure Messages with my mother's AIM account info. The result was that screen sharing worked.

What is the problem with screen sharing within my mother's origninal OS X user account?

What must I do to restore my mother's ability to share her screen with me?

We both have iMacs running 10.8.2 Mountain Lion.

Thank you for any insights.

Mark