Midi/Key editor problem

Hi , I’ve been using Cubase 7 on Macbook Pro and having an issue with the Key Editor.

Usually I like to click on several events in the Project Window (command, click), then hit Enter and all of the midi events will show up in the Key Editor.

They all show up…but on playback, when the cursor gets to a new part/instrument all the parts disappear and I can only see one instruments’ midi notes.

I’m not sure what I did but I think I accidently pushed some sort of Key Command and now I can’t see all the midi parts when I press replay. (I also wanted to view my History…but I closed the project to reboot and now the history is erased.)

This is really important for me to see chords/melody at the same time in multiple parts. I’ve searched through the Preferences dialogues and the manual but still have not found the problem.

Anybody know what I’m doing wrong??
Many thanks Jeff