There are several reasons why this could be occurring or a combination of several. The four most common reasons are:
- Insecure content, which means there are HTML elements on your site being explicitly linked by http. This would need to be updated via your system administrator.
- Missing or invalid intermediate chain. Your certificate is issued from an intermediate file. Make sure that you install this alongside your certificate on your server. If you do not have this file please contact your SSL provider.
- Your certificate is issued with the SHA-1 hashing algorithm. Browsers no longer trust this algorithm. You will need to reissue with SHA-2.
- It is the incorrect certificate. Sometimes your old expired certificate or a certificate provided by your hosting company or a self-signed certificate is installed on your site. You will need to identify the source of the incorrect certificate and contact that party to resolve the issue.