Hi there,
Are you able to share a link to your site?
If you don’t want to share it publicly yet that’s fine, but makes it a little harder to troubleshoot ??
I assume you’re using the Portfolio custom post type added by the Jetpack plugin, correct? Please go to WP-Admin ->Appearance ->Customize and click on Content Options. Under Featured Images there, is Display on Pages checked?
That setting affects the display of featured images of both the page itself, and of any posts or portfolio projects added to the page via a shortcode, so you need to have that option enabled for the portfolio project thumbnails to work.
https://jetpack.com/support/content-options/
Also make sure your shortcode includes the argument image=true
(though “true” is the default value for that shortcode if you don’t include the image argument, so that should not strictly be necessary).
https://en.support.wordpress.com/testimonials-shortcode/#attributes