I have a peculiar problem with aero peek. It sometimes just decides not to work anymore, and only shows labels instead of a preview. This is only temporary, because when I switch between other windows, the aero peek is activated again. I actually reproduced this behavior by switching to this post, and writing for 1-2 minutes. Hovering now only produced labels/tooltips. By switching to another window, aero was "activated" again.
I have tried the "Find and fix problems with transparency and other visual effects", but Windows couldn't find any problems