All the nodes must be resolvable from the host running the HMC process.
if you are certain that the entries in your host file that you uploaded exactly match the host names in your /etc/hosts on ALL hosts, then your name resolution service may be configured to hit an external DNS source BEFORE the local /etc/hosts
a good test is to manually ssh into the host running HMC
then manually ssh into each target node and ensure you can do so with the same name in the /etc/hosts
and with no password