1) Have you waited long enough? A switch in the config takes at least 1 minute.
2) Are there enough pageviews on the website in the meantime, as the cron gets only executed in wordpress if there is a pageview.
3) Do you use any caching plugins in wordpress? As plugins like w3totalcache and other plugins that use a .htaccess redirect to temporary created html files can prevent that the wordpress cron work.