Why it happens
Yorkshire puddings rise because the batter is full of liquid that turns to steam in a very hot oven, puffing up the batter before the egg and flour set around it. If the fat in the tin isn't hot enough, the batter sits in it and soaks it up instead of starting to cook and rise straight away. Opening the oven door lets heat escape, so the steam collapses before the puddings have set. A batter that's too thick or too thin, or an oven that's not hot enough, also stops them rising well.