Had this happen to me a couple times. It hits the stall and just sits there. I've seen a normal size pork shoulder take 20 hours.
This seems to happen with particularly fatty pork shoulders. Its essentially an extended stall that is cooling your pork as fast (or faster) as the smoker can heat it.
One time, I was having a get together with a bunch of people, and had 6 pork shoulders on my smoker. 5 of them finished at somewhere around 10-12 hours, the last one was a trooper and was STILL GOING at 19 hours, was only at around 175 internal temp. I ended up pulling it off the smoker, wrapping it (i never wrap my butts), and throwing it in the oven at 375. It finished in about an hour after that.