There are a few reasons why. It's normal for a German pancake to deflate shortly after coming out of the oven, similar to a soufflé. This version doesn't get as puffed up as a classic Dutch baby because of the oat flour. If your Dutch baby didn't puff up at all, it could be because your oven wasn't quite hot enough. Make sure you are keeping the oven closed as much as possible to retain the heat.