Why Are My Cookies So Flat When I Bake Them?

My wife used to make the best oatmeal chocolate chip cookies, but something changed over the past two months. Every time she bakes them, they come out extremely flat. We can't figure out what is wrong.

A few months ago my wife made the best batch of her oatmeal chocolate chip cookies that I've ever had. I asked her what she did differently, but she said she hadn't changed anything. Ever since that day, all of her chocolate chip cookie batches have come out extremely flat. They collapse...with no thickness at all to the cookies. We have experimented using different cookie sheets and even bought new flour...but all of the batches for the past couple of months have collapsed...and they even taste like they are missing an ingredient. Trust me, we are making sure we are carefully following the recipe. I miss my thick, crunchy cookies. What am I doing wrong?

Additional Reporting by Kiah Heron