A website may not rank because Google cannot properly discover, crawl, index, understand, or evaluate the page for the target query — or because competing pages better satisfy the search intent and show stronger relevance. Rankings are not a single switch. Indexation, technical access, keyword fit, content usefulness, and competition all interact. The right work depends on which of those barriers is actually in the way, which is why diagnosis should come before a generic SEO package.
Common expansions of that answer include indexation gaps, keyword and intent mismatch, thin or unhelpful content, technical SEO issues, weak architecture and internal links, thin topical coverage, tough competition, trust gaps, a recent migration, or a poor page experience. Start with diagnosis, not a generic SEO package.