I'll see if I can find out anything about the Kawai MP6-- I'm downloading the manual now.
Note that when you use a MIDI controller with a DAW, the DAW usually has a MIDI learn feature that lets you map a particular control's messages to a particular VST parameter or DAW action. Your DAW might have a built-in template or whatever for your cheap 49-key controller, but might not have one for the Kawai MP6, so you might just need to use the MIDI learn function to set things up.
Another thing I just thought of is that the knobs or other controls on a keyboard might actually be sending the same CC events as each other, but for different MIDI channels-- i.e., where each MIDI channel is used for a different instrument voice (e.g., the RIGHT1 and RIGHT2 voices if two sounds are layered together, and the LEFT voice if the keyboard is split, etc.). Often when you add a new MIDI track or virtual instrument track in a DAW, the track will be set by default to accept events from all available MIDI inputs and on all MIDI channels, which would mean that a CC event meant for channel 1 will have the same effect in the DAW as the same CC event meant for channel 2, or channel 3, etc. I'm not saying I think that's happening in your case, but I thought I'd mention it in case you ever do run into that sort of situation. I always change the MIDI input options on tracks to select a specific device and a specific channel for each track, unless for some reason I want to combine the events from two or more devices on a particular track.
If I see anything promising in the Kawai's manual, I'll let you know.
EDIT-- Addendum:
I take it you're referring to the four assignable control knobs shown on page 10 of the manual? If so, which assignment(s) are you using-- Effect, EQ, Tone Modify, or Assign? Chances are the CC messages sent by the controls will change depending on which assignments you use, and in that case you should be able to map the same knob to multiple functions in a VST depending on the assignments. Also, some assignments might not send MIDI events at all-- e.g., the EQ assignment probably controls the keyboard's internal EQ and might not output any MIDI events.
See pages 32 and 33 for instructions on how to assign different CC events to the four control knobs.
EDIT #2-- I forgot to say, I happened to notice on the Kawai site that there was a system software update for the MP6:
Kawai Downloads - Download Archive - Page 3
I don't know if your MP6 is up to date, but you might want to check it out just to be sure.