Do you give the oven a really good pre-heat before it goes in? Is it flopping inside the oven or after you bring it out? Maybe you're not cooking it ho enough or long enough? How do you check that it's cooked, with a skewer or with your finger?

DH was a chef in a former life and he said it's usually because the mix is too moist, but if it's a sponge cake and you bang the door, it can flop.