Taskbar items when grouped won't pop-up list

I have XP Pro SP2 with ALL updates. Just recently my taskbar stopped popping
up the item list for items that group together (like 10 sessions of IE).
When I left click on the group icon nothing happens. Everything else
functions as expected including right clicking on the group icon. What I
expect to happen is that I get a vertical menu of each program so I can
select one. What has gone wrong and how do I fix it. I know I could expand
the toolbar and have one icon for every program or just run fewer programs at
once but that seems stupid. I've tried rebooting and resetting the Group
function in Properties, but nothing so far has worked.