Drag and drop issue - drag item needs to always be 'in front'

Hi heroes,

I have a drag and drop which requires learners to drag items to form a diagram. Therefore some of my drop targets are 'stacked' on top of each other. This also means that I need the drag items to always appear stacked in a certain order when they are dropped, no matter which order the learner drops them in. The issue is that if they drop to 'top' drag item first (the 'CAP' in the attached storyline file), the other drag item sit on top and hide it when it is dragged on top.

I have attached the storyline file and any help with this would be awesome. I'm thinking maybe a state change for the item that blocks it, based on trigger conditions - but I do wonder if there is an easier way.

Cheers.