Monomic not catching every button press

Hello.

I am trying to capture button presses from my Monome 40h, but I cannot seem to capture them reliably. I am using a very basic sketch:

import processing.serial.*;
import jklabs.monomic.*;

Monome m;
void setup() {
  m = new MonomeSerial(this);
}

void draw() {
  if(m.isPressed(0,0))
  {
    m.lightOn(0,0);    
  }
  else
  {
    m.lightOff(0,0);    
  }
  
  m.lightOn(5,5);
}

The final m.lightOn(5,5) is simply there to confirm the connection works. It causes the correct light to turn on, so I am sure the monome is working correctly.

The problem is that it the the sketch only responds to about 1 in 20 button presses. Any idea how to diagnose this?