Recently I've been having difficulty getting my cakes to come out fluffy all the way through. They're coming out with a dense line about 1/4-1/2" thick near the bottom of the pan. I don't know if it has to do with my oven, or the position of the rack, or the time the batter is beaten, or the duck eggs I'm using, or the phase of the moon, or the way I hold my mouth while baking. I would really appreciate any insight before having to experiment with all these different variables. Thank you!