I left a chiffon I baked on the counter, wrapped in tin foil. And after 5~6 hours it shrank and became stiff. I thought maybe it's too warm and dry in the room(25 C, humidity around 35 %) so I put the next one I baked in the fridge. It became stiff anyway. And the similar thing happens to all other cakes involving whipping egg white, but not to store-bought cakes. Is it natural for cakes to shrink so fast? And is there something I can add to the cake to prevent this? We can hardly finish an 8-inch cake in one day!
Check the protein content of your flour. Make sure it is not too high. Also, did you add too much sugar? Do not use confectionery sugar. And try using corn oil in your next attempt. Hope this helps.