I use the /assist leader macro then push the Interact with unit key and it works on NPCs only every other time.
I bet you some good beer money that you don't have "click-to-move" enabled on your drones (you don't need it on your main).

It's under the Mouse options in the game settings menu.

If you don't have it enabled then your drones will only be able to interact with targets that are in range. If you enable it, they will actually put themselves in range and then interact with the target.

I swear this must have been added by a Blizz developer who multi-boxes - it's an absolute God-send