Why won't the Taskbar hide?

I've got the taskbar set to autohide, but sometimes it won't. Sometimes when this happens one of the icons in the taskbar has a yellowish background instead of blue, and it's this application which is requesting attention - once I've opened this window and done whatever it's asking me to do, the taskbar will autohide again. But often none of the icons is highlighted. If I've got 20 or 30 windows open, how am I supposed to find out which one is stopping the taskbar from autohiding? If it won't autohide, it's normally covering the horizontal scroll bar or some other important feature of the window I'm trying to use, so them I have to make the window of this application smaller. Eventually it seems that autohide is more hassle than it's worth - I might as well have the b*** taksbar there all the time.

Any ideas on how to overcome this annoyance?

Thanks - Rowan

have a look here for the moment Microsoft Fix it Solution Center
i had this prob and just searching my archives
some programs running can cause the taskbar to not close.
run the pc from fresh boot (stop autostart programs )with out unessential programs running then introduce the ones you want to run one by one and see if there is a particular one that stops the taskbar auto hiding.
have a look at brinks tutorials Tip
Microsoft has made available a hotfix that will resolve a problem where the taskbar customizations are lost and reset to defaults when automatic logon is enabled in Windows 7 and Windows Server 2008 R2. More details are available at FIX: The Taskbar is reset to the default settings when you use the "Automatically Log On" feature in Windows 7 and in Windows Server 2008 R2.
http://www.sevenforums.com/tutorials/2200-taskbar-auto-hide-turn-off.html