First thing's first, make sure you're explicitly setting qb_supervisor to be the name or ip address of the supervisor. This should be done on the image. At the end of the day, that setting will go into /etc/qb.conf. Along those same lines, be sure that qb_domain is the same on every machine. The default value for qb_domain is "qube" and should remain that way unless you are managing multiple farms on the same network.
Be sure your firewall/iptables is turned OFF. If you can't turn them off, be sure you add the correct rules for Qube to work through your firewall:
http://docs.pipelinefx.com/x/zoJThe next thing to look at is the "Banned Hosts" list in WranglerView:
http://docs.pipelinefx.com/x/iYF. You said you cleared the ban, but to be sure, check the banned hosts list.
After that, look at the worker's workerlog to see if there are any error messages. I suggest look at the workerlog from the bottom (last line) up.
Lastly, you should look in the supervisor's supelog. Look for the hostname and/or ip address of the machine that should be joining but isn't. If there's an entry there, there will likely also be a reason as to why it wasn't able to join.
If you're running Qube 6.5, be sure you're running at least Qube 6.5-2 which officially supports CentOS 6.5. Qube 6.5-3 is the most recent & recommended version as of this writing.