Can't SSH into VMs on compute nodes

Hi all,
i have installed OpenStack on 7 nodes :
 1 Controller node : Nova-* / keystone/glance/ horizon
 6 compute nodes : nova-network, nova-volume and nova-compute
Fixed Network : 10.0.0.0/20
Floating Network: 157.159.100.33/20
Evrything seems fine but when i launch a vm on a compute node, i can't SSH into it. Port 22 is already open and working but after i do this : ssh -i priv_key ubuntu@157.159.100.33, it asks me for a password for the ubuntu login !!
i tried a lot of other images ( cirros, tty linux, ..) but still the same result.
Note that injection of key to VM is done flawlessly check : http://pastebin.com/TPxsk8UP
Note also that i can access to my VMs using their Fixed IPs instead of the floating ones !
Note that SNAT / DNAT seems good

I have installed OpenStack on a single Node, used the same images and it worked like a charm, so to narrow this down, is there an extra configuration i should for my compute node ?

PLEASE HELP, I HAVE TO DO A DEMO for my boss very Soon , Thanks