Why Watch dog timer program is not working for UN0 and Nano?

While testing watchdog program with uno and Nano, I noticed that it resets but it fails to reload again, may I know the solution for this ? I tried programming using USBasp, upload 3different Arduino bootloader,but it is not working.

#include <avr/wdt.h>

void setup(){
Serial.begin(9600);
Serial.println("Setup started :");
// make a delay before enable WDT
// this delay help to complete all initial tasks
delay(2000);
wdt_enable(WDTO_4S);
}

void loop(){
Serial.println("LOOP started ! ");
for(int i=0; i<=5; i++){
Serial.print("Loop : ");
Serial.print(i);
Serial.println();
delay(1000);
wdt_reset();
}
//infinity loop to hang MCU
while(1){}
}

Please Explain what library you are using and what you are trying to achieve.

first of all you check led is connected correct
and what is time delay you set for watch dog time ?