There are two things that usually causes cron failures:

1. It runs from a different directory than expected
2. Different environment variables, usually PATH is rather minimal in cronjobs.

Check those two issues, and change your program to produce useful error messages on failure, so that cron will send you a mail with the error message.

Update: and be sure always to edit your crontab with crontab -e, it will warn you when the syntax is wrong.