Rest the dough. Thirty minutes is the minimum. Rolled straight after kneading it springs back and you will fight it into shape rather than roll it.
The filling has to be cold. Warm filling softens the pastry from the inside, and a soft pie tears as you seal it. Spread it out on a plate to cool it quickly.
Thick, not wet. If the filling still looks loose after the uncovered simmer, stir in the tablespoon of cornflour slaked in a little cold water and give it a few minutes more. A wet filling will not let the edges seal.
170°C, two at a time. Hotter and the outside colours before the pastry cooks through; cooler and they drink the oil. Frying more than two at once drops the temperature and does the same thing. Go by colour rather than the clock.
They are a hot food. The pastry softens as it cools, so eat them within the hour if you can. You can bake them instead, brushed with oil, but they come out closer to a pasty than to the fried street-food version.