When you're trading with villagers if you have the EXACT amount that they are requesting, sometimes it will take a random amount of that item you're attempting to trade with them, and not give you anything in return.

Sometimes when you trade with them, it just doesn't trade, but it does not show the "X", indicating that they're not willing to trade this.

In this case if you watch the 41 second video, it will show you the second thing that occurs.

https://www.youtube.com/watch?v=VGvFBqfVjBo