Here's a couple things, from my personal experience, nut is a little harder to run than pea, especially in the warmer weather. You really need to give it some air. The smaller the coal size, the lower the temp at which it will burn.
Your stack temp shouldn't be all that hot. I can put my hand on my stack for maybe a 2 count. If the stack is hot, then all your heat is going up the chimney.
If your air intake is closed, how are you keeping the fire going?
IMO, the nut coal is going to give you fits. Try a bag of pea and see if it's better.