High Altitude baking/cooking

I am having a problem.  I live 3000 feet above sea level.  I have been baking sourdough bread with a starter that is well over 100 years old.  The flavor of my sourdough bread is awesome, but the texture of the actual bread (inside) is too dense and finely textured.  I have tried everything to create more gas inside the bread, but to no avail.  What am I doing wrong?  If my altitude is too high, how do I compensate?  Suggestions are welcome!