Cubase 7 and vst synths problems

Hello I have a strange issue with cubase 7 and plugs like nexus or kontakt

If I load up 5 or more instances of the same plug playing rough the same midi notes.
Some of the tracks don’t play any sound. If I solo them they come back to life.

I have win7 16gb rme fire face and intel 9380k CPU so no power problem.

What could it be?

It sounds like the polyphony of the plugin is being maxed out. You may have too many notes playing at once.

I didn’t know Cubase had a polyphony.

I’m using different inst tracks. How is it possible the Polyphony is maxed out? There are tracks not playing at all.