The following code will redirect all to https://www..... Make sure you are not using any other Wordpress plugin or configuration that could generate a conflict with it.
RewriteEngine OnRewriteCond %{HTTPS} offRewriteRule .* https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]RewriteCond %{HTTP_HOST} !^www\. [NC]RewriteRule .* https://www.%{HTTP_HOST}%{REQUEST_URI} [L,R=301]