Sponge cake batter too runny
Making stiff not dry egg whites is very important in any chiffon/angel food cake that is counting on the expanding hot air to give lift to the cake. I worked as a professional baker for 18 years. When making egg whites for chiffon and other such cakes we used a combination of salt sugar and cream of tarter using a planetary mixer. At home now I use 1/8 tsp cream of tarter 2tsp sugar and 1/4 tsp salt per two egg whites but do not exceed 1tsp salt at any time unless you cut back in the batter so as not have a too salty cake. Be very careful that the bowl is perfectly clean. Any type of oils will prevent your egg whites from whipping satisfactory Using a cold bowl and allowing your egg whites to be very cold also helps as the natural friction will break them down (like whipped cream). Also be careful with how aggressive you are with the folding. If you knock it down to much it will not come back. Hope this was of some help.