so the coins should be at least 4 days old?
Maybe this is the reason. I have coins in my wallet for a long time, so when I unlock wallet, it start to staking immediately. I assume somebody who know it for sure will answer soon with details.
Cof Cof ... QED
AFAIK, right now specification are:
Min age: 2 Hours
Max age: 1 Day
Minted Blocks Maturity: 60
TxConfirm : Min 3 block
Taking a practical example, after receiving the coins by someone
else, they are confirmed and become expendable after minimum three confirmations.
After 2 hours, they become mature and ready for the staking, and their weight
(and so the probability to stake) grows for another 24 hours.
So passed the first two hours, _when_ the wallet is on line, it have mature coin,
it is synchronized, and is "unlocked" or "unlocked for staking only",
it will show a green arrow, and it could generate a new block stake, but only if
you are very very lucky, and on line.
Usually it takes a lot more, depends on the global network weight, depends on
the POS difficulty, depends on the value of the pile of coins, and as said before
depends on luck.
And if luck has a fixed limit of two hours, there is no limit to bad luck,
there may be pile of coins that have not produced stakes for several weeks.
Obviously there are extreme cases, normally expect a few days.
In any case, after having produced one stake block, the pile of coins becomes
unavailable for 60 blocks, while waiting to be confirmed.
After that, it again becomes expendable, and after two hours, become mature to
generate another stake.
Each pile of coins that you have received in wallet has a story in itself,
and can produce stake independently of the other pile.
It is not uncommon to have two or more pile that can go in the stake,
while others is still in the process of waiting for the confirmation.
Bye
! 
