I just had a thought, if you candled the eggs and some were a little slow could you not move them into the middle of the nest for a day and set them there just turning side to side? That way they'd get warmer than the outer eggs and develop more evenly? This is akin to what a broody hen does.
I was thinking the same thing...you should feel the difference between the heat at the center of the nest compared to the heat at the perimeter...those eggs feel just slightly warmed. The eggs at the center are heated not only by the heating pad above but by their fellow nestmates, making the very center eggs the hottest in the clutch.