I have run into an issue twice where I can not get some NPC quest givers to respond, ie. I do not get a "[X] Talk" prompt when I get close to them. First it was Kjar (in Windhelm) and then Wylandriah (in Riften). In both cases I used to console to start the quest but after I completed all the stages they would still not respond. The engine knew the quest was active as it put the arrow over them for the last stage, but there was no dialog option.
Ideas ?