TM4C129XNCZAD: TIVAWARE UART drivers - Eight characters before interrupt

Trying to use Tivaware Uart drivers.

After configuring a port, I don't get an Rx interrupt until eight characters have been received then get them all at once (then have to input eight more to get next interrupt, etc).

Transmit seems to work fine.

Here's how I set the port up (base and UartVector are correct values, tried without ROM prefix and does same, it is running under TI_RTOS):

        Hwi_Params params;
 
        Hwi_Params_init(&params);
        params.arg = chnl;
        params.priority = 0x80;
        params.instance->name = "Uart";
        Hwi_create(UartVector[chnl], Uart_Isr, &params, NULL);

        ROM_UARTConfigSetExpClk(base, 120000000, baud, (UART_CONFIG_WLEN_8 | UART_CONFIG_STOP_ONE | UART_CONFIG_PAR_NONE));
        ROM_UARTIntEnable(base, UART_INT_RX);
        ROM_UARTEnable(base);