WordPress does only generate the image in different sizes when you upload an image.
Therefore you have to generate your existing featured images after you have switched themes or changed the image sizes. A simple way to do that is by using the Regenerate Thumbnails plugin.
You can download the plugin on wordpress.org/plugins/regenerate-thumbnails or simply search for ‘Regenerate Thumbnails’ on your WP Dashboard → Plugins → Add new and install the plugin right from your WordPress admin panel.
After you have installed and activated the plugin, you can regenerate your thumbnails on your WordPress Dashboard → Tools → Regen. Thumbnails.