you are talking about SE? I don't remember ever seeing anything in a mouseover other than the opening post. Are you running this in "collapsed" mode and refering to that instead?
With the old format, a mouse over would show some text of all the posts in a thread. The new format is only showing text for the OP. Placing the mouse on the title of any post below that does nothing. One needs to click on the title to bring up the message body. That title is a shade of gray so learned behavior might see it's color as being a sign that it's inactive.