I found this response from customer care about mechanics of chests on Gameloft forum: 

We cannot control the items received by each player. This function is like a lottery in which it's all about chance.

Opening chests gives you a chance to win one of many items and it does not in any way guarantee that you will win the one you wanted.

Please note that some items are easier to obtain than others, but this is intended and it's a part of the mechanics of the game.

Thank you for your understanding.

Note that you are not supposed to receive the reward appearing at the moment you click. The reward you will receive is absolutely random.

If it happens to receive the item you clicked on, this is a coincidence and not the way the chests work.

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