STM32 Interrupt Priority

Question

When using Rx interrupt, I transmit the data to another place via SPI during the interrupt. However, while this process occurs when there is a delay in the main loop, the process does not occur when there is no delay in the main loop.

What would be the reason?

Kind regards

void HAL_UARTEx_RxEventCallback(UART_HandleTypeDef *huart, uint16_t Size){
    HAL_UARTEx_ReceiveToIdle_IT(&huart3, RxBuffer,RxBufferSize);
    size1=Size;
         send(1,RxBuffer,size1);

}

  while (1)
  {
 if(getSn_SR(1)==SOCK_ESTABLISHED){
 if((size = getSn_RX_RSR(1)) > 0){ 
    recv(1, TxBuffer, size);
    HAL_GPIO_WritePin(GPIOB, GPIO_PIN_1,GPIO_PIN_SET);
     HAL_UART_Transmit_DMA(&huart3, TxBuffer,size);
    HAL_GPIO_WritePin(GPIOB, GPIO_PIN_1,GPIO_PIN_RESET);
    size=0;}}
//  HAL_Delay(100);  
  }

When it enters the interface, I want it to return to the main loop without doing the operation there and I want to give priority to that side.