When you mint you r stake of coins that did the minting are held for a number of blocks.
If you look at your transactions window it will tell you how many more blocks you need to wait for your Zeit to become available again.
if you want to always have a minimum of Zeit available for transactions, you can set a reserve balance and your wallet will always then leave that amount available.
I think from the console you use the command 'reservebalance XXXX'
Where XXXX is the minimum balance you want to keep available