Can you establish a manual connection between the instances by running Elasticsearch on one instance and using telnet 10.XXX.XXX.XXX 9300 on the other instance to check the connection? (Port 9300 is the default port used for inter-node communication). Have you properly set-up the security groups? See "Step 6: Configure Security Group" at https://www.elastic.co/blog/running-elasticsearch-on-aws