Freeform drap and drop failure

I have a very strange situation. I created a drag and drop slide. When I previewed the slide, everything was fine. But after I publish the course and view the course, even though I dragged the correct shape to the correct drop area, I got the incorrect feedback. After that, I changed the setting of drag-and-drop to return the item to starting point if drop outside and published it again. I found when I tried to drag an item to the drop area, the item would always return to the starting point. That means the system could not find the drop area.