Drag and drop only drops items into slots in ascending order then stops working

My drag and drop is working by changing the image of the button I am hovered over when I let go,
I can drag an item from slot 0 to 1, 1-2 2-3 and so on but if I try to drag and drop an item in descending order it doesn’t work, the slot where the image of the item is supposed to be just goes blank
any solutions?
Also is there any way to debug my drag and drop operation an option to only appears after I’ve dragged and dropped an item?

Here’s a video of the problem, still haven’t found a solution and any help would be much appreciated - YouTube