Plugin does not work in custom theme

By the sounds of it, your custom theme is missing the common hooks that allow plugins to alter/output their code.

To take a simple example, every theme should have a call to wp_head() somewhere in the section of the output page. This allows a plugin to “hook” into your , and for example, output code to load its Javascript.

Here’s a real-life example. The WordPress Twentyfifteen theme has this in its header.php file (traditionally the part of a theme that outputs the section of any page):

... other <head> stuff
    wp_head();
    wp_footer();

?>  
</head>