Make sure to serve your tarte tatin soon after inverting it as the caramel sauce will continue to soak into the pastry, causing a soggy crust. There are also a few other reasons for a soggy tarte tatin; excess liquid from the peaches being too ripe, or undercooking the caramel and not giving it enough time to thicken. Also, make sure the tarte has been baked long enough to thicken and for the pastry to be nice and crisp.