[Yaffs] YAFFS - directories dissapear

Hi,

I'm using Yaffs2 on a 256MB nand flash chip. I have encountered following problem:

When I put files on the nand flash via ftp, all seems to go right. But when I turn off and on the power of my device, and connect back via ftp to it, I see some directories/files are disappeared. Although I could see them before I turned of the power supply.
Doing a umount/mount before turn power off/on, doesn't change a thing.

Did someone experienced the same issue? Where to look at?

Kind regard,
Stijn.