This usually happens when your cake pan is too small for the amount of batter. This actually happened to me while testing this recipe. I was testing the cake in an 8x2 inch round pan (like I do for most of my almond flour cakes) and it was sinking ever so slightly in the middle. Once I switched it to an 8-inch square pan, which is slightly bigger, it rose up perfectly. If the size of the pan isn't your issue, the culprit could also be that the oven door was opened too soon to check on the cake, the cake was not baked long enough or the batter was over mixed. All of these things can result in a sunken or deflated cake.