It could fail in any of the reason listed below:

1. It fails if there are some issues with DNS like configuration or expired.
2. ".htaccess" is not accepted due to the missing configuration in ". Wellknow" folder
3. Due to some IP block on server or issues with letsencrypt server down.
4. Domain that are pointed to Cloudflare.
5. Java or node using port 80 won't get the ssl active.
6. If the domain is disabled, suspended or the directory does not exist, will cause SSL issues.

If the issues still there, you can manually run the script below to renew it.