Cause for Popovers not Rising in Oven?
I agree with the "fast rise" requirement. An additional suggestion: it might have been not the oven, but your muffin tin. 

A hot oven is not enough for pop overs, you need well preheated containers. Originally, you would use iron ones, not just a muffin tin. If you do have to go with the tin, choose a metal one, the ones with silicone cups just don't hold enough thermal energy to give to the dough. 

Be generous with the fat. It is not only the tin which makes them rise well; being dropped in sizzling hot fat does them good. The pastry brush suggestion doesn't sound too well; it might work, but you are on the safe side if you melt a bit of lard in each cup. It is even better to get it heated on the stove, it gets hotter than in the oven. 

And a different suggestion: The temperature of the batter is important too. They tell you to use warm ingredients, and that is good. Too cold and you won't get the popovers to steam internally quickly enough. But be aware that you shouldn't make it too hot. If your milk and/or butter is hot enough to cook the eggs while mixing, the batter won't rise. Best to use all warm ingredients in the 40-50 degrees celsius (100-120 fahrenheit) range.