Your WELCOME for the WELCOME!!
First, 6300 ft isn't high to me. Not that it won't effect things. I live at 5000 and work up to 10,000 regularly. Those elevations will alter things for sure but not enough from my experience to cause problems.
Second, make sure your joint prep is good and clean inside and out. You could be pulling contaminants from inside the pipe.
If your flow is a little too high, above 25 scfm, at that elevation and coming through some cooler temps currently, you could be frosting up the regulator and hindering gas flow. Also make sure there are no kinks in the gas supply line. I generally run at about 15 scfm. Too much can actually cause it to move on out of your weld area too fast. Are you using one of the gas lenses? They will help with gas distribution.
Wrong tungsten, may be a consideration. Pure tungsten works for Alum and other non ferrous materials. Try one of the others...Ceriated or 2% Lanthianated. Hopefully others will help out here. I'm still using up my 2% Thoriated which works well on Carbon Steel but has health and other draw backs compared with the newer electrodes.
Have a Great Day, Brent