?? Sorry, I have no idea what you are talking about. Please explain.

Are you talking about Knobs (MODX has but 4) or Sliders (MODX has but 4)?
If they work when they are tasked with controlling 1-4, and 5-8, it follows that they are the same when doing 9-12, or 13-16. Nothing physically changes in the Knobs or Sliders.

Any “lag/stutter” ? issues is a MIDI issue. You’d better provide information about your VST and DAW, etc., etc.
As to your MIDI Monitor, no doubt the data is coming from the MODX, virtual synths have no physical Knob or Sliders, pretty much guarantees any lag or stutter is a function of the receiving devices through MIDI routing. (error)

You can test your Knobs and Sliders on the MODX as follows:
Disconnect the USB cable (very important for this test).
Recall the Init > “Multi/GM” Performance
Test your Sliders and Knobs by *selecting* a Part, each-by-each.

Once you convince yourself there is no lag or stutter…
How are you connected to the computer? USB or through a MIDI interface?
Are you using any adapters or go-between?
What VST?
What DAW?
What type of computer?
Using which driver? Specifically what’s the Audio Interface? (The MODX, or something else? ).
Are you using the REMOTE mode to communicate with your DAW and VST?

Let us know.