To be eligible to stake and mint, your coins have to become 'mature'. JMC coins are mature to stake after 7,75 days. That is the reason why minimum staking period is 7,75 days.
So after being in the wallet for 7,75 days, your coin are ready to mint, and there is a chance every 90 seconds (JMC block time) you get a stake.
You need to keep your wallet open and unlocked to make crypto, but if coins are not matured (under 7,75 days), you can close your wallet. This applies to each individual line (transaction) in your wallet.
Guide the cursor on an icon 'Arrow up' in the right-hand bottom corner of the wallet to see if your coins are mature or not.