Does your IWT work, when your toon is standing in the NPC's face?
Or does the IWT not work, when right on top of the mob?

If the IWT does work, when in close proximity...
You likely need to enable Click to Move (CTM).
That is in Interface Options - Mouse Options, check the box.
You can also toggle it on with (1) and off with (0).
/console AutoInteract 1

If the IWT does not work, even in close proximity...
Then the windows which don't work aren't receiving the keybind.
Possibly the mapped key is not set to broadcast that to the other windows, or some such.








The default IWT mapped key, that the IS Boxer wizard makes for you...
- Turns on CTM
- Sends IWT to the targets
- Turns off CTM

If you use this, CTM will be off regularly and will therefore fail in any one-step mapped key that wants to include IWT imbedded in another action.
But is also very convenient, in that you don't have your toons running about, from clicking on the map.

So its a toss up on how you want to use this.