Snap Face Not Working on objects? (.gif)

Hey everyone. Sometimes the snap seems to work well for me, other times it doesn’t.
In this example I’ve copy and pasted a box, and want to put it exactly on top of the other.
I use G for move, Z to lock to axis, and face snap is enabled, but no luck.

Any idea why this might be?

Maybe try a bit of this https://www.youtube.com/watch?v=7FzE0RMbqSE