Why does my chiffon cake separate when I bake it?

I have tried the Baking Illustrated recipe for making chiffon cake several times. I follow the instructions faithfully (or so I think), but every time my cake separates into a thin, gummy layer at the bottom and an otherwise normal and delicious cake layer. It's a mess because this usually causes the cake to stick to the pan and total destruction when I try to get it out of the pan. I can never present it to guests even though it tastes delicious. Please help!