SEO Auditing and Optimization

We have a website. It has long been well ranked for its subject: buying gold in Moscow. However, over the last two years, Yandex has taken us out of the top for some rather meaningful phrases. With information that Yandex in the latest updates mixes in the top irrelevant sites familiar, but the composition of the top of our subject has not changed much. Hence the concern that he may have a reason for this.
We need to find possible reasons that make our relevance to the queries worse than others. And then correct everything that was found.