Does your Netlify hosted website show up as insecure when visiting your domain with the www prefix, but it shows up fine without www?
Go to Domain Settings > HTTPS. If you only see yourdomain.com listed there, that's your problem. It should say *.yourdomain.com, yourdomain.com.
The quickest fix is to 1) delete your domain on Netlify, 2) add your domain again, 3) apply for a new SSL certificate (under the HTTPS tab).
A few minutes later, you should have HTTPS enabled on both your www and non-www domain.
I don't know why the problem occurs in the first place, but it has happened twice on my end, and this was the solution both times.
