It is possibly whatever you are using as shortening. Butter melts at a lower temp than shortening and can result in flatter cookies. If shortening is old, it loses some of its properties as well. In addition, some recipes are designed to turn out flat cookies. These are typically the ones that have you drop batter onto the cookie. Thus, it may be NOTHING that you are doing wrong, but may merely be the recipe that you are using. I wish I could be a bit more helpful but, unfortunately, I am what is known as "flour-challenged" so try not to bake more than I absolutely have to. There are some good bakers here, maybe one of them can give you better advice.
"The odds of my being correct on any given issue are inversely proportionate to the proximity of my wife" BubbaGourmet