Puff pastry will not puff if it goes into the oven when it is warm at room temperate or if the oven temperature is not hot enough. After preparing your dish with puff pastry, put it back in the fridge for an hour or so to chill. Chilling will firm up the fat in the puff pastry, which will help it to puff when it goes into a hot oven. As well as being well-chilled puff pastry also needs a hot temperature to rise, so always preheat the oven to 200⁰C/400⁰F before baking.