A co-worker ran across what may be the same problem a while back. Which is that cron does not constantly check whether the crontab has been changed.

Try setting it for a cron in a half hour or so. Or wait a day and see if it runs.

Oh, and another warning. Cron is infamous for not starting with necessary environment variables set up. Tracking this down can be annoying. See RE (tilly) 3: Get default login environment for a solution to that problem.