Problem on same priority tasks

I am working with atmega128. I am using FreeRTOS v10.4.1. I have created two tasks with the same priority and tested them on Proteus , but the switching between two tasks occurred just one time and did not repeated again. I have configUSE_PREEMPTION and configUSE_TIME_SLICING set to 1 in FreeRTOSConfig.h.

#include <avr/io.h>
#define F_CPU 8000000
#include <util/delay.h>
#include "FreeRTOS.h"
#include "task.h"

void task_1(void *pv);
void task_2(void *pv);

int main(void)
{
	DDRA = 1;
	DDRB = 1;
        xTaskCreate(task_2,"task2",85,NULL,2,NULL);
	xTaskCreate(task_1,"task1",85,NULL,2,NULL);
	vTaskStartScheduler();
}


void task_1(void *pv)
{
	while(1)
	{
		PORTA ^= 1;
		_delay_ms(50);
	}
}

void task_2(void *pv)
{
	while(1)
	{
		PORTB ^= 1;
		_delay_ms(50);
	}
}

I would check to see if the tick interrupt is working, I am presuming that _delay_ms(50) is calling eventually calling vTaskDelay (or something similar).