Flew today several bomb runs in B-17s. Last mission I did my eastern heading alt was 20.5k calibration and grd spd spot on and target were hit precise. On return heading west alt 18k with cal and grd spd perfect bombs hit approx 1/4 mile aft of target. This happened for 4 remaining Targets, (Dars at 4 different bases) and none came close to target. Been flying for years and only fly bombers, I DO NOT MISS MY TARGETS, and this was confounding. Going to restart AH to see if it will reoccur.