Arduino not reading nextion display sliders

Hi! I am trying to use a nextion display with an UNO, and I came across a problem, that I don't understand. I want to simply read the value of the slider, whenever I "release" it. Here is my code:

#include "Nextion.h"

#undef DEBUG_SERIAL_ENABLE

NexSlider h0 = NexSlider(0, 3, "h0");

NexTouch *nex_listen_list[] = 
{
    &h0,
    NULL
};

void h0PopCallback(void *ptr)
{
    uint32_t retval = 0;

    bool succes = h0.getValue(&retval);

    if(succes){

      Serial.println(retval);

   }else{

      Serial.println("Could not read slider!");

   }
}


void setup(void)
{    
    nexInit();

    h0.attachPop(h0PopCallback, &h0);
    
    dbSerialPrintln("setup done");
}



void loop(void)
{   
    nexLoop(nex_listen_list);
}

The code can be compiled, but no matter how hard I tryed, it always sas:"Could not read slider!"
The grounds of the display and the UNO are connected, and the communication is working.(I have tryed a simple button, and that worked!)

Can anybody help? Thanks!