I have an older Duncan electric kiln, and seems to be functioning fine outside of one problem. The firings seem to be taking longer and longer each time I fire. All elements are working (glowing) and the kiln does get to temperture eventually. But what took 8 hours to fire 6 months ago now take 12 hours. Not sure what would cause this if the elements are fine. Do elements degrade and lose their ability to create heat with time? Or is it another problem I'm not thinking of?