why is ask to share someone screen greyed out when on yosemite, sometimes it is greyed out, other times it's not

can someone tell me when I try to screen share and ask if she will let me see her screen, the option to ask to screen share is greyed out. The weird thing, is, is sometimes, it's greyed out, and sometimes it's black. Can not figure out why that would be. Tried same thing with my Bro and the option was not greyed out.

Is there a glitch or what when its inconsistent about the option to ask to share their screen.