AJAX call to admin-ajax.php by subscriber returns home page

I have also faced same issue,

after checking my functions.php codes,I found following code.

add_action('init', 'blockusers_init');

function blockusers_init() {
  if (is_admin() && !current_user_can('administrator')) {
    wp_redirect(home_url());
    exit;
  }
}

This code redirects the ajax call to home page.

To resolve this, add DOING_AJAX condition,

add_action('init', 'blockusers_init');

function blockusers_init() {
  if (is_admin() && !current_user_can('administrator') && !defined('DOING_AJAX')) {
    wp_redirect(home_url());
    exit;
  }
}

Hope this will helps you.