Short answer: usually cold ingredients, under-creaming, over-mixing the flour, or an oven that’s too cool.
A dense sponge has lost its air somewhere along the way. Start with butter and eggs atroom temperature — cold ingredients won’t hold air and can curdle the batter. Cream the butter and sugar properly, 3–4 minutes until pale and fluffy, because that’s your rise. Then fold the flour in gently and stop as soon as it’s combined; over-mixing develops gluten and knocks the air out. Finally, check your oven’s actually up to temperature — too cool and the sponge sets before it can rise.
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