Things to check:
- all servers have the correct time, and are synchronised using NTP. This includes backend servers running cron, and the memcached server
- nothing else is using the cache and causing it to be cleared
You are using a very old version of Moodle - Moodle 2.7.20 was released on 8th May 2017 and support for it ended on that date. Moodle 2.7.0 was released in 2014 and is over five years old. You are no longer receiving any security updates.