As opposed to the amount of butter, as Avian Flight mentions, it might actually be the temperature of the butter (or the shortening, or whatever) when the cookies go into the oven that makes the difference. I remember seeing on one of those cooking shows on the Food Network (I think it was Good Eats?) a discussion of the perfect cookie, and one of the tips was to make sure your butter wasn't too soft. I think I somewhat proved this fact to myself once when I over-microwaved a stick of butter to soften it, ended up with completely melted butter, and my cookies came out of the oven much thinner than usual.
Since you kind of need to soften the butter so you can mix it in, you could try chilling the dough for maybe ten minutes before popping it into the oven. This will give the cookie dough enough time to solidify in the baking process before the heat of the oven causes the butter to become runny. Don't over-chill, though, or you'll end up with the opposite problem: tall cookie mounds that don't bake all the way through in the center.
I was going to say the same thing! I have made the same mistake of microwaving my butter and the cookies came out completely flat.