I have been baking pastries for 45 years....and have had a problem that I can't figure out....any advice would be welcome. I have been using the same recipe for about 20 years and this last month for the first time the cake comes out sunken in the middle and does not rise to the usually height. I have now tried this cake 6-7 times this month. I bake and sell cakes on a regular basis so none of the ingredients are out of date. Cake is a basic poundcake....12oz butter, 2 1/4c sugar, 6 eggs, 3c flour(use reg unbleached), 3/4c milk, 1T b.powder, 1t vanilla, 1t almond ext, 1T lemon zest. I am sooooo frustrated. When I'm mixing the ingredients the only thing i have noticed is after adding the milk the mixture seems to break a bit, but beating seems to bring it back together.