Obviously 6400 of your coins generated stake and now you have to wait 520 blocks before they'll be available for spending again. That's how stake blocks (and stake rewards) are generated. If you want to make sure this doesn't happen you have to set the reserve balance field in the ppcoin.conf file. Someday they plan to put this control into the GUI.
Does it mean the coins that have not generated stakes will not show in the "Stake:" amount in the ppcoin-qt GUI?
I do not enrypting my wallet. I have these in my ppcoin.conf
datadir=xxxx
server=1
rpcuser=xxxxx
rpcpassword=xxxxx
daemon=1
reservebalance=1
I have some 20 some coins that are more than 30 days old and I am wondering why the "Stake:" remains 0. So the reason is just that my coins haven't found POS blocks?
30 days is the minimum age for generating stake coins. They are not guaranteed to generate stake at 30 days, it might be 31, it might be 50, it might be 450000, but the older they are the more likely they will find PoS blocks.