Launched new instance on EC2 with same keypair, can't ssh - ssh

Question

This is on AWS. I tried to launch a new Ubuntu instance (#2) with the same keypair that is associated with another Ubuntu instance (#1).
AWS instances are associated with keypairs at the time they are launched and cannot be changed once launched.
I can connect to instance #1 but can't connect to instance #2 using the same ssh command. I've checked my permissions and everything seems to be ok.
The key is in ~/AMI.pem.
I have chmod 600 on my home directory and chmod 400 on ~/AMI.pem, so it's only readable/writeable by me.
The instances are nearly identical, with the same security groups and keypairs, same version of Ubuntu, etc. Instance #1 has some additional packages installed. I verified both are associated with the same keypair called "AMI".
Connecting to #2:
USAU9900:ScrapeEdgar reedn$ ssh -i ~/AMI.pem -v ubuntu#54.67.18.96
OpenSSH_6.2p2, OSSLShim 0.9.8r 8 Dec 2011
debug1: Reading configuration data /Users/reedn/.ssh/config
debug1: Reading configuration data /etc/ssh_config
debug1: /etc/ssh_config line 20: Applying options for *
debug1: Connecting to 54.67.18.96 [54.67.18.96] port 22.
debug1: Connection established.
debug1: identity file /Users/reedn/AMI.pem type -1
debug1: identity file /Users/reedn/AMI.pem-cert type -1
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_6.2
debug1: Remote protocol version 2.0, remote software version OpenSSH_6.2
debug1: match: OpenSSH_6.2 pat OpenSSH*
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug1: kex: server->client aes128-ctr hmac-md5-etm#openssh.com none
debug1: kex: client->server aes128-ctr hmac-md5-etm#openssh.com none
debug1: SSH2_MSG_KEX_DH_GEX_REQUEST(1024<1024<8192) sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_GROUP
debug1: SSH2_MSG_KEX_DH_GEX_INIT sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_REPLY
debug1: Server host key: RSA 36:69:f6:95:b4:3c:f0:77:02:ab:02:01:af:c7:cc:6a
debug1: Host '54.67.18.96' is known and matches the RSA host key.
debug1: Found key in /Users/reedn/.ssh/known_hosts:1
debug1: ssh_rsa_verify: signature correct
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug1: SSH2_MSG_NEWKEYS received
debug1: Roaming not allowed by server
debug1: SSH2_MSG_SERVICE_REQUEST sent
debug1: SSH2_MSG_SERVICE_ACCEPT received
debug1: Authentications that can continue: publickey
debug1: Next authentication method: publickey
debug1: Trying private key: /Users/reedn/AMI.pem
debug1: read PEM private key done: type RSA
debug1: Authentications that can continue: publickey
debug1: No more authentication methods to try.
*Permission denied (publickey).*
Connecting to instance #1:
USAU9900:ScrapeEdgar reedn$ ssh -i ~/AMI.pem -v ubuntu#54.183.77.100
OpenSSH_6.2p2, OSSLShim 0.9.8r 8 Dec 2011
debug1: Reading configuration data /Users/reedn/.ssh/config
debug1: Reading configuration data /etc/ssh_config
debug1: /etc/ssh_config line 20: Applying options for *
debug1: Connecting to 54.183.77.100 [54.183.77.100] port 22.
debug1: Connection established.
debug1: identity file /Users/reedn/AMI.pem type -1
debug1: identity file /Users/reedn/AMI.pem-cert type -1
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_6.2
debug1: Remote protocol version 2.0, remote software version OpenSSH_6.6.1p1 Ubuntu-2ubuntu2
debug1: match: OpenSSH_6.6.1p1 Ubuntu-2ubuntu2 pat OpenSSH*
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug1: kex: server->client aes128-ctr hmac-md5-etm#openssh.com none
debug1: kex: client->server aes128-ctr hmac-md5-etm#openssh.com none
debug1: SSH2_MSG_KEX_DH_GEX_REQUEST(1024<1024<8192) sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_GROUP
debug1: SSH2_MSG_KEX_DH_GEX_INIT sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_REPLY
debug1: Server host key: RSA a6:53:a0:99:b8:61:ef:fe:05:d3:fe:47:90:ea:de:b6
The authenticity of host '54.183.77.100 (54.183.77.100)' can't be established.
RSA key fingerprint is a6:53:a0:99:b8:61:ef:fe:05:d3:fe:47:90:ea:de:b6.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added '54.183.77.100' (RSA) to the list of known hosts.
debug1: ssh_rsa_verify: signature correct
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug1: SSH2_MSG_NEWKEYS received
debug1: Roaming not allowed by server
debug1: SSH2_MSG_SERVICE_REQUEST sent
debug1: SSH2_MSG_SERVICE_ACCEPT received
debug1: Authentications that can continue: publickey
debug1: Next authentication method: publickey
debug1: Trying private key: /Users/reedn/AMI.pem
debug1: read PEM private key done: type RSA
debug1: Authentication succeeded (publickey).
Authenticated to 54.183.77.100 ([54.183.77.100]:22)

Answer 1

First you need make sure the keypair has been applied to instance #2. Something like below in aws console on this instance:
Key pair name: AMI (should be same as instance #1)
Then try below steps:
If this ec2 instance is not in VPC, could you stop and start the instance to refresh with new public IP address and try again?
clean content in .ssh/known_hosts in your local machine, and try again.
One of them should work for you.