Quick answer: the three common causes are (1) mixed content, where images or scripts still load over http, (2) DNS that has not updated yet, so the certificate cannot be issued, and (3) your site not redirecting to https. Check them in this order.
1. Mixed content
The page loads over https but some images, styles or scripts use http links. Change those links to https. In WordPress, set the site address to https and use a plugin such as Really Simple SSL to fix old links.
2. DNS has not updated
The certificate is issued only after your domain points to your hosting. If you changed nameservers or records recently, wait 1 to 2 hours (sometimes up to 24) and check again. You can see the current DNS at dnschecker.org.
3. No redirect to https
Visitors who type http:// may stay on the insecure address. Redirect http to https in your website settings, or ask our support team to help.
Still stuck?
Open a ticket with your domain name and a screenshot of the warning.