This can happen for a number of reasons, but it is typically due to the store's primary domain not being configured correctly. When you go to Online Store > Domains in the Shopify admin, are you seeing any error messages appear here such as "SSL pending" or "SSL failed"? Also, can you please confirm that you are not trying to implement an SSL certificate provided by a third party provider on your Shopify store, and are just using the SSL certificate provided by default to all Shopify stores?
If you are comfortable, please also share your store's URL here and I'd be happy to take a closer look.
Victor | Shopify Social Care