Why did my Popovers deflate?
The Secret to Perfect Popovers - Kitchen Conundrums with Thomas Joseph
If your popovers lose volume when they come out of the oven, they are probably underbaked. When these airy baked goods aren't cooked enough, too much steam stays trapped inside. That moisture condenses once they're removed from the oven, causing them to collapse. The perfect popover, however, is easy to master.