Hi david.danaan,
Yes the featured image is not displayed on single page. We have designed our theme this way. You can add images on the post editor from media. If you like to display the same featured image on single page then you have to add some code at theme folder -> library-> structure -> content-extension.php on line no 258 inside this functions attitude_theloop_for_single()
but adding direct code on the theme, you will get problem while updating to new version. So we always recommended you to create a child theme so that while updating to new version your customized code will not be lost. If you are not familier with child theme then each and every time while you made code customization, the same changes need to make on the theme.
<?php
if( has_post_thumbnail() ) {
the_post_thumbnail();
} ?>
Thank you!