There's sometimes a bug, that makes shifts the clicking area one half a cell up and right.
So if you can't click the NPC try clicking the square that is 1 up and 1 right of him. This might help.
Usually it get's autofixed by restarting the game or sometimes doing a hyper jump is enough.
Also Make sure that you can see the NPC, I mean you can't talk to him through a wall or a closed door :P