To interact with a target your characters need a target. So I think the problem lies in assiting. How do you make sure your slaves have a target? If this is the problem, first casting a spell or manually targetting something should show this. Since if they do IWT if you switch windows, then the problem lies in assisting.

Also, you're sure they keycombination get's broadcasted to all characters? And that they all have the same keybindings so it does get broadcasted?