It's been happening for a long while and I suspect it would involve a big change to at least make it more manageable. My theory is that the loot frame or whatever frame you are trying to look at the tooltip for is layered behind another object. Usually the dead body in regards to the loot window. Twisting the screen around so that there is nothing in the way usually lets it work. Not great but hopefully that feature will be missing off of version 6
