Search runs in three distinct stages. Diagnosing a problem is mostly a matter of working out which stage it failed at, because the fixes have nothing in common.
1. Crawling
Google discovers URLs from links, sitemaps and previous crawls, then fetches them with Googlebot. A page can fail here by being undiscoverable (nothing links to it, no sitemap), blocked, or too slow to fetch reliably.
2. Indexing
Google processes what it fetched: renders the page, reads the content, works out what it is about, and decides whether to store it. Duplicate detection happens here, several URLs with the same content get collapsed to one canonical. A page can be crawled and still not indexed, which Search Console reports as “Crawled, currently not indexed”.
3. Serving
When someone searches, Google selects from the index and orders results by relevance, quality, usability and context. Being indexed guarantees eligibility, not visibility.
Alien Road recommends: name the stage before you propose a fix. “We are not ranking” is not a diagnosis. Search Console tells you whether a URL is crawled, indexed, or indexed-but-not-appearing, and each of those points to a completely different piece of work. We start every technical engagement with that classification.