If it is constant, Options -> UI Settings -> Tooltip Enable (1/4 of the way down from the top)
If it is sporadic, my best advice is to re-start the client. If that does not help, look at your Network Status Display Anything out of the ordinary? Loss Rate should be 0.0%. Recv: Is generally in the hundreds of Bps. and not thousands of tens of thousands (although the number can spike when loading into the game or a new zone, loading bank data, during combat, etc. Lastly, try moving your cursor over the object and then remove your hand from the mouse. I have had instances where the slightest movement of the mouse causes the window to immediately disappear or to not appear at all.
I have also had issues where the Tooltip for an item (inventory / bank / chest / et al.) continues to flash on screen for a brief moment (1/10 of a second or such). In this case I have had success linking the item in a chat window and then clicking on the link.
My 2 cents, although worth considerably less.