Ajax : Call undefined function plugin_function() … can’t call any plugin function

I use a Ajax query to show some content, but I can’t use any of my plugins functions.

Javascript :

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$('#load_more').click(function() {
    var data = {
        action: 'load_more',
        id: $("input#id").val()
    };
    jQuery.post(ajaxurl, data, function(response) {
        $("#content").append(response);
    });
});

PHP in functions.php :

function load_more(){
    $id = intval($_POST['id']);

    if(!isset($id) or !is_numeric($id)){
        echo "Error";}
    else {
        $works = plugin_function($id);
        wordpress_function_works($id, $works);}
    die;
}
add_action('wp_ajax_load_more', 'load_more');
add_action('wp_ajax_nopriv_load_more', 'load_more');

The Ajax works fine, it can call & work with any of my wordperss functions. But when I call a plugin function, it returns an error Call undefined function plugin_function() line...

PS: ajaxurl = /wp-admin/admin-ajax.php

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