I don’t know if any of these will help, but here goes.
Over beating will force air out of the cake batter. I’ve always been taught to beat the wet ingredients thoroughly (but not to over cream the butter/sugar at first), just mix through the dry ingredients until they’re uniform, and fold them through rather than beat them in. Sifting the flour is what adds air to the mix.
All ingredients should be room temp or cooler when baking.
The batter should be mixed and then immediately go into the tin and then the oven without any delays.
Make sure the oven is fully pre-heated before putting the cake tin in.
Check your oven seal - a batch of cookies can show you any hot spots or cool spots in the oven.
Line with paper or dust flour on the bottom of the cake tin, but just butter the sides. This will help the mix ‘climb’ the sides of the tin without sticking in the tin. Run a knife around the sides before tipping onto the cooling rack.
Too much baking powder can have the same effect as none at all. Make sure you’re not adding it to self-raising flour. My brother did that and ended up with hockey pucks instead of scones.
According to one of my cookbooks, old flour will not rise and only fresh flour should be used for cakes. I’ve never had that problem - but baking goods don’t get the chance to age disgracefully in this house!
I’ve found that the following microwave recipe is very forgiving (I’ve used oil instead of butter and other random mistakes). If you haven’t got a microwave cake ‘tin’, use a safe bowl with a glass inverted in the center.
Mix in one bowl - 1 cup flour, 1 cup sugar, 3 Tbsp cocoa (or more - go on, more!), 2 tsp baking powder, and a pinch of salt. I often use a whisk to mix these dry ingredients, just to add air.
In another bowl, melt 3oz butter and add 2 eggs, 1/2 cup milk and 1 tsp vanilla, beat the crap out of this (with the whisk you already have on the bench) then add it to the dry ingredients and mix through gently with a rubber spatula or spoon.
Microwave on high for 6-8 mins depending on your machine (I do 8 mins in a 750w oven). Leave it in the tin for a further 5 mins before turning out onto a cooling rack.
Dribble with chocolate sauce or orange syrup or whatever you like.
If this doesn’t work, I’m afraid you were cursed by the cake fairy and there’s nothing you can do. On review, some of these have been mentioned, good luck and please report back.