[Yaffs] nandsim and mkyaffs2image problem

Hi:
I use  mkyaffs2image to make the img,and use  the nandsim tool  to mount  
it on fake /dev/mtdblock/0,
but the file is lost.Is there some one comfront  the same problem?
  Here is my step :
    mkyaffs2image  fsdir yaffs.img
    dd if=yaffs.img of=/dev/mtdblock/0
    mount -t yaffs  /dev/mtdblock/0 /mnt/