Why is Messages screen sharing icon grayed out?

Question

Trying to do remote screen sharing with my mom's Mac using the new feature in Yosemite.
If you're not familiar with it, the new method does not require AIM, Jabber, etc and is independent of screen sharing settings in System Preferences (per Messages Help).
The procedure is:
In the main Messages window, select the name of the person you want to share with or, if you haven't previously exchanged messages, do so now.
Click 'details' on the right of the top bar of the main window.
In the menu that pops up, there are three icons: screen sharing, video chat, and audio chat/phone call. Click the screen sharing icon and send a request.
Today, my mom signed into her Messages account and we successfully exchanged iMessage messages (not SMS).
When I clicked 'details' the screen sharing and phone call icons were grayed out (or, more accurately, 'blued out' - they were a fainter shade of blue).
I clicked another name in the Conversations list and then clicked back to my mom's name. When I clicked 'details' this time, the screen sharing icon was no longer grayed out, so I sent a request to share her screen. She didn't receive the request, so I canceled it.
When I clicked on 'Details' again, the screen sharing icon was grayed out again. As far as I know, nothing had changed on my mom's end.
What could cause the screen sharing icon to be grayed out? And what could cause it to toggle back and forth between grayed out and normal?
Both computers are running the latest version of Yosemite, 10.10.3. Mine is a 2009 Mac Pro 8-core with 16 GB RAM, hers is a MacBook Pro from around 2012 or 2013.
Thanks!
Bob

Answer 1

Hi,
(He did say that he'd heard that due to changes in Google's system that their accounts no longer work with Messages.)
What he has heard and what are true are two different things
Logged in Google account
There are issues with Messages using SSL to login to the Google Talk server and Google saying SSL is Less Secure (there is a  setting to Enable Less Secure apps like Mail that also uses SSL and Messages).
Some people find that 2 step Verification cannot be set at Google's Account settings if your want to use the Less Secure Apps setting.
1. I deleted my AIM account from Messages and had my mom do the same. The result is that we each only have two items in the Accounts window of Messages: an iMessage account (signed in with our Apple IDs) and a Bonjour account.
2. Clicking on the Details menu, the screen sharing icon was available for both of us. It was not grayed out.
This is using two different Methods of Screen Sharing in Messages (It is silly it has two but there we go)
For AIM use the Buddy List and the icons at the bottom or highlight the Buddy and use the Buddies Menu
DO NOT USE the Details option.
For iMessages start text chat then use the Detail Option an NOT THE BUDDIES Menu.
For the iMessages Method you have to Enable System Preferences > Sharing > Screen Sharing to allow the App to be Invoked.
(It is like FaceTime is invoked but only works to Macs)
As you mention the Connection Doctor only refers to the older "iChat Method for Aim and Jabber and Bonjour - if you can Video you should be able to Screen Share with that method.
9:49 pm      Wednesday; April 29, 2015
​  iMac 2.5Ghz i5 2011 (Mavericks 10.9)
 G4/1GhzDual MDD (Leopard 10.5.8)
 MacBookPro 2Gb (Snow Leopard 10.6.8)
 Mac OS X (10.6.8),
 Couple of iPhones and an iPad