Elevation won't make much of a difference. How much head space do you have in the keg and did you purge it before carbonating? Head space is important because CO2 adsorption rate depends on the surface exposed to CO2.
Remember one unassailable statistic, as explained by the late, great George Carlin: "Just think of how stupid the average person is, and then realize half of them are even stupider!"
"I would like to die on Mars, just not on impact." Elon Musk