Sinking in the middle is a common issue when making individual chocolate cheesecakes. This is usually caused by the air bubbles that form during baking, which leads to an uneven surface. To prevent this, try tapping the tray with the cheesecake cups on the counter before baking to release any built-up air and help set the filling evenly. Once baked, chill the cheesecake for at least two hours before serving for best results.