That does sound strange... The fact that you hear the note you press on the midi controller yet still see several keys highlighted green on the virtual keyboard but don't hear them. Do you have anything else in the project that your midi controller could be triggering when you play a note?
The email to Steve Duda sounds like a good idea in this case.
One last thing if you have not tried this yet. Create a new project in sonar, instantiate Cthulhu Vsti. When/if asked, select a midi track and the first audio track.
Route your midi controller output to the Cthulhu midi track input.
Route the Cthulhu audio track out to Sonar master buss or whatever your sound card in connected to.
Ensure the speaker icon is lit on the Cthulhu UI. (so you are using the internal tone generator only)
Dismiss the Arp module so it is not visible
Select a chord preset.
Play a few notes on your midi controller one note at a time.
Do you hear anything?
Do you still hear a single note, the one you are pressing on your controller, from Cthulhu internal tone generator?
Do you now hear the chord from Cthulhu?
Mesh, Please don't feel obligated to run that test but, Please let us know what you find out from Steve.
Kind regards,
Rick