Most on-page audits catch the obvious stuff: a missing title here, a duplicate meta description there. The thing that quietly costs you rich results is structured data that exists but is invalid, and most flat-list crawlers either skip it or bury it. Here is why it happens and how to catch it.
The problem, concretely
You add FAQ schema to a product page to win that expandable rich result in Google. You paste a JSON-LD block into the head, ship it, and move on. Six weeks later the rich result never showed up, and nobody knows why.
The usual culprits are small and silent:
A @type that does not match the content (FAQPage with no mainEntity).
