The left [ drivers ] side on pre-magnum small blocks oils the rockers from the # 2 rocker shaft support. Consequently the # 1 & 3 intake rockers are first in line to get oiled. If all the other rockers on that side are getting oil it almost has to be some sort of blockage either in the oil hole in the shaft for the # 3 intake or in the rocker arm itself. The fact that there are more threads showing on that rocker may indicate some other problem. Usually all the adjusters will show similar thread exposure. A different style rocker or a short pushrod could cause this. Or possibly a cam lobe / lifter going away. Hope that's not it. Good luck.