OK bungie second time this has happened to me I was one achievement away from moments of triumph seal which was the armour that I didn't do now I take that as my own fault, but I have grinded for this undying seal I was one overlord kill away from it and it just would not spawn for me in archers line, I did this countless times to no avail just the vex retreat every time so now again grind for nothing what are you going to do about this?