Let's encrypt not autorenewing


i do have the issue that on some servers my certs are not autorenewing at all.
Time is set to 2 month - OS is Centos 6.8 - and i have see it on Debian 8.6 as well.

My question is:
How is that renewal triggered?
Should there be a cron job or something like that?


Yes, that would do the trick. Create a cronjob and you are all set.