Talking to a very nice lady at the Love Food Festival, she was saying that her cakes often 'sag' in the middle. This could be due to a number of reasons:
The oven temperature was too hot.
The cake was not cooked long enough.
The oven door was opened too soon, which created a draft.
It can be that too much baking powder was added. Often thinking you want a cake to rise more, with enthusiasm you can add more baking powder, but there is an optimum amount. Often when you do this the cake rises brilliantly in the oven (you proudly pat yoursefl on the back) and when it is taken out it slumps down (disppointment ensues...) The lovely lady talked about using a cheap baking powder, and that could be the cause, and that goes back to always using the best quality ingredients.
If the Cake is Dense and Heavy these could be the cause:
The eggs were too small. Always use large eggs when baking. I obviously prefer free rnage eggs :)
Insufficient air was whisked into the egg and sugar mixture.
The flour was not folded in gently. Always mix in the flour at the lowest speed.
The melted butter was too hot when added, causing it to sink down through the whisked foam.
See baking is a minefield for disasters, but so rewarding whem you get it right so worth persevering.
Use the best quality ingredients that you can.