I'm going to take a stab in the dark here, but do you have your slave open up the quest giver (right click on them)?

You need to do this on slaves as WOW security restrictions prevent you from interacting with a quest giver until you manually right-click on them to open their initial dialog.

Sorry if this isn't the issue, but on my own addon, I get this question a lot and this is often the missing piece to the puzzle.