There was a message “Staking is active” in my wallet yesterday

There was a message “Staking is active” in my wallet yesterday. But after I received 90 generated coins I get a message “Not stacking because you don't have mature coins” Why?

Every time you receive staking reward coins, the 24 hours coin age resets. You will require to wait another 24 hours to mature again and qualify for staking reward.