I have the same issue you have under certain circumstances. If the NPC I'm turning the quest into has multiple quest and I haven't completed them all, the NPC is also a merchant, innkeeper, etc and the most common reason in Cata and MoP zones is that the NPC's seem to "change" (for lack of a better term) after the master has turned in the quest and the slaves won't interact. Normally you'll see some type of dialogue from the NPC or they will be have moved on the masters screen but not the slaves.

To the best of my knowledge there isnt anyway to break that in ISBoxer so if it's only happening from time to time then more than likely it's something I described above. If it's happening all the time and you're in lower level areas then I'm not sure what the problem could be.