DFPlayer + WeMos

Hi
I'm trying to run a simple mp3 track when a button is pressed.
Unfortunately this is not working without the mp3_next() and the delay() functions...

Can anyone help understand why?

thanks


#include <SoftwareSerial.h>
#include <DFPlayer_Mini_Mp3.h>
#define BLYNK_PRINT Serial
#include <ESP8266WiFi.h>
#include <BlynkSimpleEsp8266.h>

// Your WiFi credentials.
// Set password to "" for open networks.
char ssid[] = "xxxx";
char pass[] = "yyyy";
char auth[] = "abcd";

SoftwareSerial mySerial(D1, D3); // RX, TX

BLYNK_WRITE(V4){
if (param.asInt()){
Serial.println("Direction is forward ");
mp3_play (3);
}
}

void setup () {
Serial.begin (115200);
mySerial.begin (9600);
mp3_set_serial (mySerial); //set softwareSerial for DFPlayer-mini mp3 module
mp3_set_volume (15);
Blynk.begin(auth, ssid, pass);
}


//
void loop () {
//static int i=1;
Blynk.run();
}