If the wallet is unencrypted, then you only need to leave it open. Coin maturity is 30 days, so it can take 30 days before they are eligible for staking. You don't need leave the wallet open if the coins aren't mature yet.
And should have no reservebalance set in the emercoin.conf, or reservebalance=0

And if the wallet is encrypted, then you also need to unlock for minting only using interface, or type in debug window or emercoind:

walletpassphrase [mintonly]

i.e.

walletpassphrase <> 999999 true