This happens when the TX FIFO buffer is FULL and the TX cannot send any character. I assumes three possible reasons.
1) CTS input is negated.
If you uses the CTS input and the input is negated, the TX cannot send any character.
2) UART is disabled.
If UART is accidentally disabled, SpiUartWriteTxData() can accept characters but no characters are sent.
3) UART clock is disabled.
If the UART clock internal or external is disabled, no characters are sent.
If possible, please attach a snapshot of the UART's configuration dialogue.
Can you attach your project?