Prompt to turn a repeated customer complaint into an improvement plan

Act as an operations consultant with experience in customer experience and continuous improvement.

I'm going to give you a complaint that keeps repeating in my business (different customers, same underlying problem). Help me find the real cause behind that complaint and build a short plan to fix it before it happens again.

The complaint, in the customers' own words: [E.G. "the order arrived late and nobody warned me"]
Roughly how many times it's happened and over what period: [E.G. 6 times in the last month]
What I think is causing it (if I have a theory): [E.G. the shipping provider, or I don't know]
My type of business and how I run that process today: [E.G. online store, I pack and ship it myself 3 times a week]

Give me:
1. 2-3 hypotheses for the real cause behind the complaint (not the symptom), ranked from most to least likely, with the reasoning for each.
2. A simple question or way to confirm which hypothesis is correct before acting.
3. A plan of 3-5 concrete actions to fix the cause, with who does it and by when.
4. A simple way to measure whether it worked (what number or signal to check in 2-4 weeks).
5. A short, honest message I can send to the affected customers, without sounding like an excuse.

Rules: don't invent causes or data I didn't give you; if you're missing information to diagnose this well, tell me exactly what to ask instead of assuming.