Spotting trace off-axis

Yesterday I was shooting .308 with three other guys. We were shooting from prone about 6 feet apart at the same two steel plates at 600 yards. Conditions were great for spotting trace for the shooter to my left. But when I tried spotting for the shooter to his left (i.e., about 12 feet to my left) I couldn't see any trace at all. So I moved one position left, and then his trace appeared as clearly as the other guy's had.

I was using a 56mm riflescope at about 20x, focused at 600 yards, and my field of view at that magnification showed the trace of shooters 6 feet left of me coming in from the left within a quarter of the field of view of the target. So I can't imagine why, while watching a target at that distance, I wouldn't be able to see the trace of a shooter just 6 feet further left of me. Is it known that trace visibility is very sensitive to being "on-axis" with the bullet? Or is there anything that might explain why I couldn't see it when my offset increased by a matter of feet?