Do you AG, or Extract? Do you come up to a full boil at any point? What is full boil for you? I can’t imagine that 600ft ASL is making that much of a difference in the boil point, especially with the malt in there, which would tend to raise it a bit.
Some things only happen at full boil, like melting of beef fat. If the pot is boiling at the bottom but only 170* at the top, the fat will stay in a lump.