I have checked several sites manually and some of them are unavailable, the site does not respond. Why?

There may be several reasons for that, the most frequent is blocking by hosting providers of certain countries, as well as the prohibition of access to certain resources/ranges of IP addresses by your Internet providers. For example, while in Russia, you may not be able to access some of the US sites.

Absolutely all countries fall into our database, as we bypass any blocking when checking domains. To make sure that any site is working, you can use the free service: https://ping-admin.ru/free_test/