STM32F3 Can't get the UART to work in this particular case

Question

I am having trouble making a very simple transmission between two STM32F3 Discovery boards. Let me explain what I'm doing.

I have Board-A connected via i2c to an LCD 20x4 and Board-B connected via i2c to a Qrobot 3D Gesture Sensor, the goal is to catch the movement from Board-B, blink some leds on the same board and send a number via UART (uart4) to the BoardA.

The problem is that although the debug shows that the transmission takes place, actually the Board-A never enters in the callback of RxCpltCallback. Before dive into the topic with more details, lets me show you the code:

This is the Receive callback of the Board-A. (Also this probram was tested receiving via Uart4 from another F3 (that F3 was sending also via uart4 but in the callback of the BushButton))

void HAL_UART_RxCpltCallback(UART_HandleTypeDef * huart)
{

switch (recvbuff)
    {

        case 1:
            HAL_GPIO_TogglePin(GPIOE, GPIO_PIN_9);
            lcd_send_string(up);
            for(int i=0;i<5000000;i++){
                count1=count+1;
            }
            count1=0;

            lcd_send_cmd (0x01);

        break; //causa l'uscita immediata dallo switch
        case 2:
            HAL_GPIO_TogglePin(GPIOE, GPIO_PIN_10);
            lcd_send_string(down);
            for(int i=0;i<5000000;i++){
                count1=count+1;
            }
            count1=0;

            lcd_send_cmd (0x01);


        break;
        case 3:
            HAL_GPIO_TogglePin(GPIOE, GPIO_PIN_11);
            lcd_send_string(left);
            for(int i=0;i<5000000;i++){
                count1=count+1;
            }
            count1=0;

            lcd_send_cmd (0x01);


        break;
        case 4:
            HAL_GPIO_TogglePin(GPIOE, GPIO_PIN_12);
            lcd_send_string(right);
            for(int i=0;i<5000000;i++){
                count1=count+1;
            }
            count1=0;

            lcd_send_cmd (0x01);



        break;
        case 5:
            HAL_GPIO_TogglePin(GPIOE, GPIO_PIN_13);
            lcd_send_string(forward);
            for(int i=0;i<5000000;i++){
                count1=count+1;
            }
            count1=0;

            lcd_send_cmd (0x01);



        break;
        case 6:HAL_GPIO_TogglePin(GPIOE, GPIO_PIN_14);
            lcd_send_string(backward);
            for(int i=0;i<5000000;i++){
                count1=count+1;
            }
            count1=0;

            lcd_send_cmd (0x01);



        break;
        case 7:
            HAL_GPIO_TogglePin(GPIOE, GPIO_PIN_15);
            lcd_send_string(clockwise);
            for(int i=0;i<5000000;i++){
                count1=count+1;
            }
            count1=0;

            lcd_send_cmd (0x01);



        break;
        case 8:
            lcd_send_string(anticlockwise);
            for(int i=0;i<5000000;i++){
                count1=count+1;
            }
            count1=0;

            lcd_send_cmd (0x01);



        break;
        case 9:
            lcd_send_string(wave);
            for(int i=0;i<5000000;i++){
                count1=count+1;
            }
            count1=0;

            lcd_send_cmd (0x01);



        break;
        default:

        break;
    }
HAL_UART_Receive_IT(&huart4, (uint8_t*)&recvbuff, sizeof(uint32_t));

}

This seems to work properly, I don't think the problem is here.

Let's look the code of Board-B: this program have to check, in a loop, some flags that stands for the gesture, when one occurs blink some leds and send via huart a number, a really simple task!

   while (1)
   {
         /* USER CODE END WHILE */
         Gesture_Data = DEV_I2C_ReadWord(PAJ_INT_FLAG1);
            if (Gesture_Data)
            {
            switch (Gesture_Data)
            {
                case PAJ_UP:                         { send(1);
                                                       HAL_GPIO_TogglePin(GPIOE, GPIO_PIN_9);                    
                                                      break;}
                case PAJ_DOWN:                       { send(2);
                                                       HAL_GPIO_TogglePin(GPIOE, GPIO_PIN_13);                                       
                                                       break;}
                case PAJ_LEFT:                       { send(3);
                                                       HAL_GPIO_TogglePin(GPIOE, GPIO_PIN_15);                       
                                                       break;}
                case PAJ_RIGHT:                      { send(4);                                                       
                                                       break;}

                /* other cases */
                default: break;
            }
            Gesture_Data=0;
            DEV_Delay_ms(50);
        }

}

In order to test via debugging and brekpoint I have created a separated function only for the trasmit (Also tried the HAL_UART_Transimit(&huart4, (uint8_t *)num, sizeof (uint32_t), HAL_MAX_DELAY)):

void send(uint32_t num)
{
HAL_UART_Transmit_IT(&huart4, (uint8_t *)num, sizeof (uint32_t));
}

When I try this code on the board, the sensore recognize the gesture and blink the correct led, in the debugging I can see crearly that I enter in the function sed() of the trasmit but nothing arrive on the other Board. P.S: every cable connecting the two boards work fine and are on the correct Pins.

What do you think of it? I can't get my mind off this problem!