While there are many reasons that your search rankings can fall, some common reasons are that the website has not been updated recently, there are few links to the page, or the page has been penalized for manipulative tactics.
Part of Google’s algorithm is QDF or query deserves freshness. Sometimes a newly published article will get a boost because it is very timely. News is one example of this. However news from a year ago often becomes less useful and relevant for users so they can fall in the rankings. Additionally, if there are long time gaps between additions of content to your site, Google will not send their bots to crawl your site as often, which often results in fewer of your pages showing up in the Google index or a decline in the rankings of individual pages.
Another major reason is that the page lacks links. Having a strong domain can help boost individual pages of your site, but if an individual page has no links, it will probably rank lower than a competing page that has a couple more links (all other things being equal). Additionally if another page has higher quality links they can outrank you as well.
Thirdly your site or page may have received a penalty for manipulative tactics that are against Google’s terms of service. Using “black hat tactics” such as link buying can have this effect and it can be a lengthy and difficult process to have the penalty removed.
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