I'm not sure you can use drush to initialize composer manager. I usually use the init script from composer manager and have had no issue with it:
This is how I typically do it.
php modules/composer_manager/scripts/init.php (initialize composer manager)
composer drupal-update --no-dev (fetch dependencies)
You can also run
composer drupal-update without the
--no-dev but I've found that composer sometimes can fail to update certain libraries within drupal core, as it expects to find .git files but these are not included in Drupal core builds.