Gladys - from Singapore, Singapore
Here are a few ideas to help you avoid this problem. Make sure that your oven is properly heated to the correct temperature before placing the cake inside; allow the oven enough time to heat up to the appropriate baking temperature listed in your recipe. You may want to try using an oven thermometer to double-check the temperature of your oven. If the oven dial and the thermometer temperatures don’t match, it may be that the oven is not heating properly.
Make sure to measure all of the recipe ingredients precisely. A cake won’t rise correctly and may fall in the middle if the cake batter is too solid or runny. Following the correct baking time in the recipe is important, too. If you take a cake out of the oven before too early, it may not have had enough time to fully rise and set in the oven. If the baking time is up and the cake doesn’t look finished, place it back in the oven for just a few minutes (make sure to keep a careful eye on the cake to avoid overcooking). To test if the cake is done, insert a toothpick or skewer into the middle of the cake and pull it out; the toothpick should come out pretty clean, with possibly a few crumbs stuck to it. If there is any wet batter on the toothpick, the cake still needs more time in the oven to bake.
Take care to not open the oven door while the cake is baking. Doing so allows heat to escape the inside of the oven, and causes the temperature to fall. Opening the door too much makes it difficult for the oven to stay as hot as it needs to in order to bake the cake. This can also cause the cake to not bake properly.
If a cake batter is over-mixed, too much air will mix into the batter. On that same note, under-mixing can cause uneven baking as well. For a successful cake, mix the batter only as much as the instructions state. I hope this helps!