First of all , are you using the same kernel version on each machine ? The 'same-kernel' refers to the version. You can build different kernel images of the same source version to meet the hardware/software needs of a given node. However you wil need toe make sure that when you install openMosix on your cluster, all your machines should have the openmosix-x.x.x-y kernel installed, in contrast to having one machine running openmosix-x.x.z-x, another running openmosix-x.x.x-y, another running openmosix x.x.x-z, and so on and so forth
When you run mosmon, press t to see the total of machines running. Does it warn you that mosix is not running?
If yes, then make sure your machine's ip is included in /etc/mosix.map (don't use 127.0.0.1 - if your machine's ip is such, then you probably have problems with your dhcp server/nameserver). If it does not tell you that mosix is not running, see what machines show up. Do you see only your machine?
If yes, then your machine is most likely running a firewall and is not letting openmosix through.
If not, then the problem is most likely with the machine that doesn't show up. Also: Do you have two nic cards on a node? then you have to edit the /etc/hosts file to have a line that has the following format
non-cluster_ip cluster-hostname.cluster-domain cluster-hostname
Maybe you used different kernel-parameters on each machine? Especially if you use the 'Support clusters with a complex network topology' option you should take care that you use the same value for the also appearing option 'Maximum network-topology complexity support' on each machine.