Yes, Teresa, it could be your leavening agent, although in the case of high altitude it might be rising too much and then falling. Try reducing your baking powder by half so that it doesn't over rise next time. We also have a comprehensive list of other tweaks that you can make to have better success at high altitude here on our website, including adjusting the baking time and temp, the amount of flour, and the amount liquid in your recipe. 

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