Each script that runs in a cron job must also run when you put the command directly at your console. If you don't have shell access then try to use the exec function of php.
Try to capture all output of the script. In the exec call you would do something like:
PHP Code:
exec('/usr/local/bin/php your_script.php some_arg1 some_arg2 2>&1', $output, $ret);
If I remember correctly, $output is an array. Each element is one line of the output. In $ret the exit code of the php process is stored. This should be 0 on success.