Having an SSL on a domain is pretty much the standard now, and many developers choose to use SSL For Free as their certificate provider. However, many people do in fact have issues getting and/or renewing certificates for unknown reasons. Here are a couple of my tips to get a certificate issued when your running into trouble.
If your certificate has accidentally expired and its failing to to issue a new certificate after clicking the renew button on the expired certs page, just forget it, delete it, and get a brand new certificate from the start page. This solves most problems.
First make sure your domain is live and your not trying to add the challenge files to a production server. Try putting a test.html file in the location and see if you can pull up the file at same domain as the certificate. If you can do this, then the URL path is correct.
also, When adding a new domain to lets encrypt only use justthedomainurl.com in the field. Do not add the http:// at the beginning or it will fail. Just use the full domain without http://.
DNS @ Record Issues from Forwarded Domains
If the domain is forwarded from another registrar to your hosting: check the DNS. Sometime in the DNS only the @ record or (A Record) is forwarded to the hosting.
This is the most common issue: You may need to ADD a record for WWW to the zone file or the WWW may be pointed somewhere else. This is because lets encrypt is trying to validate the WWW and the NON WWW @ record. Which is usually the case when one validates and the other doesn’t.
Try using the manually verify option, and check the links for the two downloadable challenge files. If one validates and the other does not, the WWW record is probably the issue.
The Redirect Check
Lastly, if your using WordPress or some other CMS (content management solution) make sure you go into settings and change the domain from http://yourdomainurl.com to https://yourdomainurl.com. If your not using a CMS, try adding an .htaccess file to your website root that directs the WWW and NON WWW to the https:// domain.