Assume you have 2 tracks in your project.
Each track has an FX on it: FX A on track 1 and FX B on track 2.
Assume you want to copy FX A over to Track 2 so that FX A will follow FX B on that track.
You move the mouse cursor to FX A in order to grab it.
[tooltip might appear at this moment]
If you don't wait for the tooltip to disappear, the slot on Track 2 you want to copy FX A into might be beneath the tooltip.
In this case, copying or moving will silently fail.
The same goes for sends.