php output Contact Form 7 – Repeatable Fields

I’m using the plugin cf7 repeatable fields and I’m creating a form which saves a xml file while pressing submit button.
I’m printing all the shortcodes of cf7 with $output .= $_POST[‘shortcode’]; but I’m not able to print any field of this plugin. Any suggestions?
That’s a part of my code:

If I’m trying to print the shortcode for this specific plugin, I’m able to get only an empty field. Do you have any ideas?

Thanks a lot!