Mini cheesecakes can sink in the middle for a number of reasons: 1. You didn't use room temperature ingredients so the change from cold to hot caused the cheesecakes to sink. 2. You over-mixed the batter so the cheesecakes puffed in the oven and then sank when they were removed. To prevent your mini cheesecakes from sinking, make sure your ingredients are at room temperature. You can avoid over-mixing by beating the cream cheese with beaters and then using a spatula to mix in the rest of the ingredients.