MMS provisioning with AWS problem (could not connect to server)

I am new to MMS and mongodb . I have provisioned an aws ec2 instance with mms . MY problem is whenever

 I am trying to access the Mongodb instance (through shell) from another ec2 instance like this :


~$ mongo HOSTNAME:PORT NO 

I am getting this error 

2014-11-27T15:20:16.653+0000 Error: couldn't connect to server HOSTYAME:PORT NO (54.xxx.xxx.xxx), connection attempt failed at src/mongo/shell/mongo.js:148exception: connect failed
Can somebody help me out with the problem.

Have you made sure that the security group allows these other EC2 machines to connect via the mongo ports? The most common cause of this error is these kinds of firewall/security group settings.