SSD
The impression I got, after reading the report and subsequently speaking to the inspector in order to express my complaint, is AAIB were pushed for resources and didn't check some stuff fed to them by the accident pilot (stuff the group had already seen from the accident pilot's legal advisors and which we had dismissed as a desperate attempt by them to reduce their client's uninsured losses resulting from the accident).
How serious an incident was it? This one obviously involves a fatality, and my experience of the AAIB is that they follow everything up a lot more thoroughly when someone has lost their life.
If the incident you have experience of wasn't quite so serious, they may just take the statements they receive and publish without much further investigation. If it did involve a fatality or serious injury, then obviously it's a pretty poor show if the AAIB didn't investigate further when you raised concerns.