Every time I bake cookies lately they're coming out completely flat. Obviously, they don't look good, are difficult to remove and burn easily. This isn't a new oven, just a new problem. Several sources I've checked have said that it's a problem in oven temperature, but I've done several checks with an oven thermometer, and the temperature is fine. Someone suggested using half butter and half shortening, but I have a problem with the idea of using shortening. Besides, I never had to before. Someone else suggested that it's a flour problem. This has never been a problem in my many years of cookie baking, so why now? Any suggestions would be appreciated.