Flat Banana - Dutch Oven with Altitude

On a recent trip I had the chance to try making some Dutch Oven baked Banana muffins.  Also some cornbread.  Both recipes that I have used in the past with great success.  Both did not raise the way they normally do.

I need some help, tips, or "rule of thumb" adjustments for higher altitude baking.  I normally cook at a 2000-3000 foot elevation.  I never had to pay attention to adjustments to compensate for higher elevations.  My recent baking was done at 7500 ft, and another at around
8500 ft.

Both the cornbread and banana muffins failed to puff up and create a more rounded top.  Help!