Q: A post I was sure would hit totally flopped. How do I actually diagnose why?

Q: A post I was sure would hit totally flopped. How do I actually diagnose why?

Short answer: separate reach problems from retention problems — they have opposite fixes.

Longer answer, walk this checklist:
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Pull your retention graph. Flat drop in the first 2 seconds = hook problem, not algorithm.
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Check reach vs. your median. Low reach + normal retention = the system never tested it; reshoot the first frame.
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Normal reach + early drop = your hook over-promised.
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Look at saves and shares, not likes. Zero shares means nothing was worth passing on.
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Compare post time to your last 5 winners.

Verdict: fix one variable, repost the same idea in 72 hours. One flop is noise; diagnose the pattern, not the post.