A return lands from the OEM with a part number, a serial, and a failure description. The quality engineer has to find the production batch, pull the station logs from the historian, cross-reference the shift log to see who was on the line, and reconstruct what the parameters were doing that day — usually four weeks ago, sometimes longer.
The 8D is due in five business days. Most of those days are spent finding the data; almost none on actually deciding what to change. The corrective-action section ends up being a paragraph the engineer wrote last quarter, edited slightly. The OEM accepts it because the format is right, but nothing on the line actually changes.
The pattern across returns is invisible. The same parameter drift, the same station, the same shift configuration shows up in three returns over six months, but nobody connects them because each one was an isolated forensic exercise.