It seems like HMC node have incorrect key to access newly added node.
You have to set the same key for all nodes, HMC should be able to communicate with all nodes with the same key.
Please, copy your authorized_keys file to all nodes you are planning to add.
Also, make sure that your sshd configuration on new node allows root access (it is disabled by default on RHEL).
Let us know if there more questions.