Snapshot the Logs for your Elastic Beanstalk environment through the online console.
When you view the logs, search for a section beginning "cfn-init.log".
In that section you'll see entries like
2013-08-30 10:25:13,517 [INFO] Command 01-ec2setcomputername-enable succeeded
2013-08-30 10:25:24,516 [INFO] Command 02-install-server-monitor succeeded
2013-08-30 10:25:30,115 [INFO] Command 03-install-agent succeeded
where 01-ec2setcomputername-enable, etc. are the names of the commands in my .config file. If the command fails, you should see the error message here too.
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