Let's Encrypt Certificates didn't auto renew

Logged on to one of the sites I host today to find that its security certificate expired - and also figured out that the certificates for most of the other domains on the server have either expired or are going to expire soon. However, when I go to renew the certificates, it says that they will be auto-renewed 30 days prior to their expiration. Is there a reason this didn't happen, or a way to find out why it didn't?

I'm running Plesk Onyx 17.9.10 on Ubuntu 18.04 LTS if that makes a difference.