To demonstrate my issue I have written a very simple program in Arduino:

A class declaration that contains a single private variable thing

#ifndef LED_H
#define LED_H
#include <Arduino.h>

class LED {
  private:
    int thing;
  public:
    LED();
    void setThing(int newThing);
    int getThing();
};

#endif

The class implementation

#include "LED.h"

LED::LED() {}

void LED::setThing(int newThing) {
    thing = newThing;
}

int LED::getThing() {
    return thing;
}

My main program

#include <Arduino.h>
#include "LED.h"

LED MY_LED;
void updateLED(LED led);

void setup() {
    Serial.begin(9600);
    MY_LED.setThing(1);
    Serial.println(MY_LED.getThing()); // Outputs "1" as expected
}

void loop() {
    delay(2000);
    updateLED(MY_LED);
}

void updateLED(LED led) {
    led.setThing(led.getThing() + 1);
    Serial.println(led.getThing()); // Is stuck on "2", it should be incrementing 2,3,4,5 etc
}

Why is the private thing variable not being updated? I would expect it would be incrementing by 1 every time the loop runs.