Please check if these forum threads are of any help:
I hope you have checked this Code Example for USBUART for PSoC5LP available at this link:
The processor may get hanged at the "while(USBUART_CDCIsReady() == 0u);" statement if the previous transfer to the PC is not done or completed.
You need to check the USB log for confirmation of the above mentioned scenario.
400 bytes can be sent through USB UART