Indexability would be my guess. I have noticed great content going unseen just because of poor link structure between important pages, accidental no indexing, and exclusion from the crawler paths in google. Before working on additional backlinks, I would ensure that google is able to reach the necessary pages properly.
In my opinion, content decay is one of the things that people often underestimate. i have encounntered websites where they continue to generate new webpages, yet the old articles get slowly ranked down due to lack of updates. In some cases, updating the content turns out to be more effective than generating a new piece of content.
There are many elements that can be easily overlooked, such as friction removal. Little problems such as annoying pop ups, unorganized design, or poor mobile interaction may not appear on SEO lists, but they do have an effect on user experience. If users are leaving your website bcz of an unpleasant experience, even top notch content will not save it.
Something that I do not think people give enough attention to is the crawl efficiency of their websites. There are many sites out there that have hundreds of unnecessary pages, broken internal links, and orphaned pages, and then they ask why it takes ages for google to crawl their important pages.
one point that is often overlooked is index bloat. If Google needs to crawl through hundreds of not relevant, out of date or duplicate content pages, your valuable pages may get overlooked. in some cases, the importance of removing irrelevant pages from index outweighs the effort to publish yet other blog post.
Strong internal linking is often overlooked, but it can significantly improve crawling, indexing, and page relevance.
Strong internal linking is one of the easiest SEO wins that many people still ignore.
