Yandex is very different from Google, when it comes to stable rankings and updates of search results. It is very important to keep in mind that while Google SERPs are never 100% stable, in Yandex not many changes happen on a daily basis.
This used to be the case with Google before as well, but it is still happening in Yandex. As I mentioned in one of my previous posts, every 2 weeks or so Yandex rolls out a big update and the SERPs change noticeably within 1-2 days. Compare the bars on the graph (vertical axis indicates percentage of change in the SERPs):
I would like to introduce you to a great tool to monitor these Yandex updates – Update Analyzer (available in English!). The tool was developed by the biggest Russian SEO company Ashmanov and Partners, which is also famous for organizing of the main Russian SEO conference Optimization.ru and a big SEM event eTarget.
The tool monitors and quantifies a number of parameters across all search engines used by Russian people, including even smaller ones, such as GoGo and Aport. While I have never figured out how to use Spam level or SEO-pressing, I use the SERP udate analyzer for Yandex on a regular basis.
Keep in mind that during the SERP update sites tend to disappear and reappear in the SERPs. If a strong competitor or yourself is missing from the search results, it is very likely a Yandex SERP update is in progress. Do not freak out. The update takes 1-2 days and after that the SERPs come back to normal. I recommend to check the updates stats every now and then to avoid such stressful situations!