Dave, I see that you're right down the road from me. It's been cold and pretty breezy at my house today. Is your smoker out in the wind?
I used to have a brinkmans propane smoker that wouldn't (couldn't) get meat to a finished temperature when it was in any sort of cold breeze. The wind would strip the heat off the smoker box faster than the burner could replace it. It would get in the range of mid 140s to mid 150s, but no further.
Can you place your smoker in a protected location? Just a thought.
Smoke going up...