Please fill out the fields below so we can help you better. Note: you must provide your domain name to get help. Domain names for issued certificates are all made public in Certificate Transparency logs (e.g. https://crt.sh/?q=example.com), so withholding your domain name here does not increase secrecy, but only makes it harder for us to provide help.
Some parts of the site you mentioned do have this same error, which is called a mixed content error. This is not a problem with the certificate or HTTPS setup, but with the content of the site—it’s loading subresources within the page insecurely over HTTP instead of HTTPS. An example would be if your HTML page includes something like <img src="http://example.com/image.jpg" /> instead of <img src="https://example.com/image.jpg" />, or the same for videos, scripts, frames, iframes, etc.
You can probably identify the specific problems with a tool like https://whynopadlock.com/ or with your web browser’s developer tools.