Im prob missing something obvious here,… Now im not sure either if this is since I did the last update to Cubase 6.0.6 but Ive just noticed that the knobs on the Cubase Stock Plugs cannot be turned by left clicking the mouse. The values can still be altered by mouse using the central scrolling wheel. Just not by left clicking the mouse button.
How Strange??? Can anyone out there shed any light???
Well, god, in the Project ‘Preferences’ you will find an option to set that will tell your mouse how to behave with plugin knobs. I think there are at least 2 if not 3 options. I can’t remember if 6.5 ignored my previous setting, but I do remember having to reset this preference. 
I think 6.5 did this to a few settings, actually, now that I think about it.
Anyway, not to worry, it’s there to be selected again.
This is in Preferences > Editing > Controls, Knob Mode. You can switch these options: Linear (this is default), Relative Circular, and Circular.