Ok I think this is what he is talking about. He wants the green to act as a platform, and if the yellow box is not on the green, he wants it to fall down. Get it?
I think your problem is the object isnt weighted at all, or maybe that you didnt position the objects up in the air (position object x,x,Y,z)
Id try moving the objects up, and adding some weight to the box.
I hope I got the explaining part right

You want the green a platform, and if the yellow isnt on the platform then if falls down. Right?
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