Sorry if I repeat it again, but I could not find this here.
I tried to use dm3xx_sd_boot-6.1 tool to flash my custom u-boot to the NAND and the board was failing to boot. At the same time it would work fine with u-boot binary which comes with the tool. As it stands out, the size of u-boot is hardcoded in "sdcard_flash/sdboot_flash_cfg.h" as "UBOOT_SIZE" define and is set to 160K but the real life u-boot can be about 250K like in my case.
Hope this will help you to save some time debugging NAND boot issues if dm3xx_sd_boot tool is used