The usual reason for this is over-whisking the batter after adding the flour. In fact, stop using the whisk and use a wooden spoon instead after the eggs have been mixed in. It is also important to make sure to measure the flour correctly. Use the spoon and level method rather than the scooping method. Spoon the flour into the measuring cup and level it with the back of a knife. Using too much leavening can also be a problem. Too much gas will cause it to expand and then deflate.