Build an Accessible Drag and Drop Interaction in Storyline

I ran into the same problem.  Switched from Slide View to Form View under in the Question Pane and noted the items with Drop Return only had no Drop Target identified.  

I find it's really important for me to name all objects in my slide or I will go CRAZY.  Once I named the objects and targets them the items were accepted.  Still need Layers and Triggers. But that problem was solved.