Element is not displayed when grouped

That happens because "Riflesso" is actually a layer. When you group it, the layer cannot become a part of the group, because it is not allowed as a child element of a group. But then Illustrator does something illogical and put the paths from inside the layer at the bottom of everything else.

What you need to do is first drag the elements out of the layer on top of the other things and then group them.