Flavor – Butter and Butter Flavored Shortening

Creaming puts the air bubbles into the mixture. The baking powder only helps enlarge the bubbles, not make them. In cookies the creaming plays another essential role, which is to help dissolve the sugar. To cream the butter keep it cool and do it for a few minutes (at 65°F, harder in the summer).

It has recently been discovered that cookie dough is different from cake batters. Sugar is part of the structure of the cookie and not just a sweetener, tenderizer, and browning agent. It forms the base upon which the fats and the starch granules of the flour are embedded. The sugar needs to dissolve for the matrix to form.

If you beat the whole thing, it will be harder to get bubbles in and you may end up overworking the dough. The cookies will end up flat and tough.

To answer this question we should turn to the oracle: http://www.goodeatsfanpage.com/season3/Cookie/CookieTranscript.htm

The relevant quote is:

"Nothing affects a cookie's texture more than the melting characteristics of its fat. Butter has a sharp melting point meaning that, uh, just a few degrees difference between a solid and liquid states. So since conversion occurs at a relatively low temperature, the resulting batter spreads prior to setting."

The problem is that shortening melts at a higher temperature than butter and so cookies made with shortening typically rise more than butter made cookies. Thus your rising problems are probably not caused by the type of fat.

I suspect that the temperature of your fat might be causing the problem. If your butter came from the fridge and the shortening was room temperature then the shortening cookies would melt faster and rise less.