Have you performed a root cause analysis before? It can be a helpful tool if you get the right people involved, and not just management. We use the 5 why process and include someone who is actually doing the task in the meeting. We then define the problem and include all the conditions from that day. From that point we list possible causes of why the issue could occur . Once we have our possible causes, we develop our 5 whys that lead us to our root cause. From the root cause you develop a corrective action plan.
Brief example (not knowing your entire process or conditions that you have in the facility)
Define the problem: One case had 2 missing sachets. We were short 5 people that day and had temporary employees on the line. We were also behind on schedule and had the line speed increased. We reviewed our inspection records and found all were in compliance. One hourly inspection was missed.
Possible causes: Lack of training, sachets dropped on the floor, inspections not frequent enough, line moving too fast
From this point you would go into your 5 whys (Why was the case short 2 sachets? Because the employee did not put them in the case Why did the employee not put them in the case? Because he was not trained
Your root cause could be you have new employees on your line and you need to review your training at hire. Preventive/Corrective Actions could be to notify them of the complaint, retrain your employees, and increase inspection frequencies.
Again, not knowing all the details around the situation I'm just giving some examples to get you started. I also attached a form that we use as an example. Hope this helps!