How do you make Yorkshire Puddings rise reliably?
While using a hot oven and keeping the tin hot while filling are both critical elements, equal concern needs to be taken with making sure that your batter is at room temperature.
If the eggs and milk aren't room temp to even slightly warm, then it will take a significant amount of heat to simply warm the batter in the pan before significant steam can build for their expansion.
You can warm eggs quickly by placing the whole egg (in shell) in a bowl and covering with hot water from the tap.  Let it sit for 5-10 minutes and you'll have room temperature/warm egg.  Milk can simply be microwaved to only slightly warm (not hot, or you'll cook the eggs).
You might also try using bread flour.  Here in the south all-purpose flour has a lower gluten content than most other all-purpose flour in the US and it's also bleached to weaken the gluten content that's there.  I've recommended to guests of mine that have had issues with popovers not rising that they try bread flour and I've heard positive responses following the use of bread flour. Bread flour will also have a bit more flavor and produce more browning from the additional protein.