Problems in using UBL - OMAPL138

Hi,

   I'm using my own board that was designed base on TI OMAPL138-LCDK, with OMAPL138B 456MHz，DDR2(MT47H128M16RT-25EIT)， NAND FLASH(MT29F4G08ABADAWP-IT) on board.

   Now, i have problems when i attempt to download UBL to Nand.

1.) When i use Nand writer in CCS, i found it can only download one file and then end. I can download thd UBL file successful, and the UBL runs to search uboot sucessful too. But i don't know how to download uboot again, then i modified the Nand writer program to start from block 6, and download uboot.bin to try, there was noting response. I tried uboot.img too, but no response too. I wonder how to download uboot using CCS? And what format? Is there any document?

2.) Unfortunately, i download uboot in AIS format directly into Nand without UBL, after that, the board can't work any more.I try to erase it by Nand writer in CCS, but the Nand writer program just run to start and no response any more. That's why ?

3.)I change to another board, and download with sfh_OMAP-L138.exe/UART Boot Host.exe, but it has problem too,like this:

(Serial Port): Opening COM14 at 115200 baud...
(AIS Parse): Read magic word 0x41504954.
(AIS Parse): Waiting for BOOTME... (power on or reset target now)
(AIS Parse): BOOTME received!
(AIS Parse): Performing Start-Word Sync...
(AIS Parse): Performing Ping Opcode Sync...
(Serial Port): Read error: The operation has timed out.
(AIS Parse): I/O Error in read!
(AIS Parse): Boot aborted.
(Serial Port): Closing COM14.

after this, i use serial assistant to simulate the boot protocol in "Using the OMAP-L132/L138 Bootloader" document, and i can receive "BOOTME" without problem, when i send Start-Word:0x58, i can receive 0x52 also, but when i send Ping Opcode:0x0B,0x59,0x53,0x58, there is nothing response again, so, does anyone can help me?

  Thanks a lot!