DM368 UART0 Problem

I'm using the DM368 chip and there are several pieces of PCB boards' minimum system
appears to not be operational:
The power supply is good;the power sequence is correct;
The chip reset is correct;The GIO0(B5) pin is pulled up to 3v3.
At UART0 Boot mode,The GIO18/UART0_TXD(E2) pin output high electrical level,
just when the power is on there are serial print information:
"BOOTME BOOTME BOOTME BOOTME BOOTME..."
but at NAND Boot mode,The IO18/UART0_TXD(E2) pin output low electrical level
all the time,so there're no serial print information in the course of nature.
I would really appreciate your help!