UART Flash download failed

Dear Sir/Madam,
We are testing the UART download of code via SmartSnippets for production preparation.
We found it is very hard to burn the code via both JTAG and UART.
We tried on 3 different types of board including our PCB board and the other 2 types of development board. The issue is the same.

Downloading process of flash_prgrammer.bin or jtag_prorgammer is very stable. No issue related to it.
The issue is related to erase or burn function. After a success erase or burn operation, most of time we got all-zero content except 5th bytes(0x80). And we also met that the random content generated when select read 32k content.
Yes, there is chance that we can burn the code into the flash correctly, but that is just rare case.

Can somebody help to clarify the correct hardware connection of UART to Flash and correct software usage?

Frankly speaking, SmartSnippets is very unstable.

Thanks!
Regards,
Simon