There are two main reasons. 1) You used puff pastry. Puff pastry can be used to make tarte tatin, but in that case, it must be served immediately. Do not let it sit for too long, or the puff pastry will absorb the moisture of the apples. 2) You didn't drain the excess juice. After baking, very carefully drain the excess caramel juice by tilting the pan at a 45° angle over your sink. Keep your hand on the shortcrust pastry to prevent the tart from flipping. Next, you can flip the tart completely over a serving plate.