Structured data that validates but never earns a rich result

Validation and eligibility are two different things, and the gap between them accounts for most of the structured data questions I am asked.

A validator confirms that the syntax parses and the required properties are present. It does not confirm that the markup describes what is actually on the page, that the page qualifies for the feature, or that the search engine wants to show that feature for the query. Markup can be perfect and earn nothing.

The failures I see most often are markup describing content the visitor cannot see, and a type that is technically valid but does not match what the page is for. The third is a page that qualifies on paper while the query never triggers the feature at all.

So my order is: does the page deserve the feature, does the visible content support the markup, and only then does the markup validate. Starting at the validator is starting at the end.

The case I cannot explain is a page that clears all three and still shows nothing for months while a weaker competitor does. Is there a diagnostic step I am missing, or is that simply the answer being no?