Why are some of my coins locked? Why does it takes so long to unlock them?

There are no "coins" in your wallet, there are "outputs" corresponding to transactions you have sent or received. These outputs can only be spent in their entirety. So if you have an output of 10 that you received and you use it to send 1, then a transaction of 9 is sent back to you as change. As with any transaction, this one coming back to you takes time to confirm. Until that time the wallet reports that 9 of your balance as locked.