1. The eggs were poorly mixed
2. The dough was mixed incorrectly
3. The oven didn't heat up to the right temperature or was cold

1. You didn't use enough dry ingredients (flour/ cocoa/ sugar).
That prevents the cake from staying in shape after being cooled down.

2. Poor quality dry ingredients.
Make sure to follow a good recipe and use high quality ingredients.

3. You mixed everything on high speed
This gets rid of needed air bubbles

4. You opened the oven in the process of baking

5. You got it out of the oven too soon

6. You used a spoon or a mixer to combine the wet and dry ingredients
Use only a silicone spatula

7. The dough isn't baked

1. The baking temperature is too high
2. You set the oven at the wrong regime
3. You placed it too close to the bottom or the top of the oven

1. It was baked for too long
2. It cooled down incorrectly or too long
3. It wasn't wrapped up in plastic foil and left in the fridge after baking
4. It was cut with а dull knife

You should mix the eggs on a low to normal speed.
By mixing them too fast, you'll destroy the air bubbles, which will result in your sponge cake flattening. Better mix them longer and it will end up just perfect.

The egg yolks and egg whites should be mixed together.
If you don't separate them, the cake will evenly turn out to be "fluffy"
If you mix them separately, there is a high chance to overmix the egg whites. If so, the cake will have a lighter color and will rise poorly.