Custom plugins don't show up right away

I have a couple of really simple plugins I've created in a custom module and tested on a development site. When I enable my module, however, the plugins don't show up, for example under /admin/config/content/wysiwyg/profile/filtered_html/edit

This happened during development as well. I cleared the cache, looked at the wysiwyg table in the database, couldn't figure out what was going on, and then mysteriously, they showed up. Now that I've enabled my module on our production site, the same thing is happening. in MYMODULE.module I have implemented hook_wysiwyg_include_directory like so:

/**
 * Implementation of hook_wysiwyg_include_directory
*/
function MYMODULE_wysiwyg_include_directory($type) {
        return $type;
}

and in plugins/myplugin.inc I have implemented hook_wysiwyg_plugin like so:

/**
 * Specially named implementation of hook_wysiwyg_plugin().
 *
 * Should be named {$module}_{$plugin}_plugin().
 */
function MYMODULE_MYPLUGIN_plugin() {
  $plugins['MYPLUGIN'] = array(
    'title' => t('My Plugin'),
    // The 'icon file' is the icon that appears in the Wysiwyg toolbar.
    'icon file' => 'myplugin-icon.gif',
    'icon title' => t('My Plugin Tile'),
    'settings' => array(),
        'extended_valid_elements' => array(
                'div[class]',
        ),
  );
  return $plugins;
}

Am I missing something that is needed to make the plugin show up upon enabling the module?