I've downloaded and setup newest yaffs2 sources on my kernel. All is fine. 
I can do 
mount -t yaffs2 /dev/mtdblock3 /mnt/y
cp test1 /mnt/y
ls /mnt/y
    lost+found    test1

but after 
umount /mnt/y 
mount -t yaffs2 /dev/mtdblock3 /mnt/y

when I do  ls /mnt/y
I receive only lost+found folder

I don't receive any warning/errors messages. Can someone help me and 
recommend what to do?

Thanks.