Two things. One, are there any conditionals in the dialog? If no dialog branch is valid, no dialog will launch. Two, are you sure your character can access the placeable, i.e., nothing is blocking it? Try moving the invisible placeable a little bit forward, because sometimes it looks like there is nothing in between you and the placeable, but the game engine thinks there is.
And I almost forgot, you did put the dialog file inside either the Override folder or packaged it into the relevant module file?
- Star Admiral