In the Prezi Edit Mode you might stumble upon a situation where to want to add a frame or an element to the path, but that option seems to be missing from the menu and from the right-click mouse menu.
The solution to this problem is quite simple – the item you are trying to add is already the last item on the path.
Drag and Drop the last slide higher in the left sidebar and now you will be able to add your content to the path again!
