We are trying to start our app with pm2 0.12.8 on ubuntu 14.04 with octa core proccessor. The read me on the git hub has a very straight forward command for running node app in cluster mode.
# Cluster mode
$ pm2 start app.js -i 0 **# Will start maximum processes with LB depending on available CPUs**
$ pm2 start app.js -i max **# Same as above, but deprecated yet.**
have you tried starting a fixed number of processes? i.e.
pm2 start app.js -i 2 //should start two instances.
what does "pm2 monit" show you?
pm2 stop all pm2 delete all
pm2 start app.js -i 0
if you stop a process in pm2 it still reserves one cpu for it even if its not running. you should allways use pm2 delete