Why Is Page Content Not Showing Up In Wordpress Custom Theme?

Though there might be a lot of problems regarding your case, I'm going with the most basic one since it's a custom wordpress theme.

Unlike posts wordpress requires a separate page named page.php to display pages. Check if you have one and make sure that's not empty which should look similar to the one shown below.

<?php
get_header(); 
	while ( have_posts() ) : the_post();
		get_template_part( 'content', 'page' );				
	endwhile;
get_footer();	

Make sure you also have a content.php file where you need to add the commands to print the title, thumbnail and excerpt. You can add html and css elements as well as other elements as per your requirement as well.

<?php
the_post_thumbnail();
the_title();
the_content();	

You can also add the comment section, links to other pages, etc. as needed.

If this answer doesn't solve your issue then you might need to send a copy of your page.php and associated files for a thorough assesment.