Drag and drop activity: items don't do what I want...

Hello everyone,

I have a problem with a drag-and-drop activity. The green shapes don't always come back to their place.

As you can see in the attached file, if you put the item "1" in the empty shape on the right, it works. If you continue in the "right" order, everything is ok.

But it's a kind of... mmh... free activity. I mean, you can drag and drop in the order you want AND you can come back to a previous item.

Example: you follow the "right" order from 1 to 4 but you want to come back to the item 3. So you put the green shape "3" on the right in front of the shape "4" and here, there is a problem. Indeed, the shape "4" don't come back to its place and the shape "3" is hidden behing the shape "4".

Why? It's like two pieces of two differents layers  appeared in the same time O_o ! I am a beginner so Storyline 360 still have a lot of mysteries for me...

(My english is as bad as my Storyline skills so really sorry for my mistakes ^^)

Aurore