How do I read all the data that should come with UART in STM32F4?

Question

I am currently working on USART with STM32 and it was working while I'm using usart interrupt. It was filling the rxBuff while I am sending with HTERM via USB but it is not filling while I am trying in the main block. It just takes first character of the transmitted data. For example, I tried to send hello, it just takes "h" into rxBuff and stops. When I try to send it again, this time rxBuff becomes [h,h] which means it only takes the first character.

Working:

void USART1_IRQHandler(void) {
    
    rxBuff[i++] = USART_ReceiveData(USART1);
    
    if (i > RX_BUFFERSIZE ){
        i = 0;
    }
    USART_ClearITPendingBit(USART1, USART_IT_RXNE);
}

Not working :

int main(void){

while(1){ 
    if ( USART_GetFlagStatus(USART1, USART_FLAG_RXNE) == SET ){
        rxBuff[i] = USART_ReceiveData(USART1);
        i++;
    }
    if (i > RX_BUFFERSIZE ){
        i = 0;
    }
 }
}