Please let me ask some questions.
I assumed that there is one UART component and one Interrupt component as follows. Is this correct?
There are several kinds of interrupt sources handled by the Interrupt component. Which interrupt sources did you select?
The UART component can have hardware or software FIFO. What RX buffer size did you select?
The API function UART_GetChar() returns ZERO if the RX buffer exhausted. Did you check if any ZEROs are contained in the data array?
Can you please post your complete project so that we all can have a look at all of your settings. To do so, use
Creator->File->Create Workspace Bundle (minimal)
and attach the resulting file.
There is simple example of using Fixed Fiction UART Rx port ( PSoC4 ) for receiving control messages from terminal. Please check if this approach fits your goals: