Not sure I can offer an answer to the “WHY” question…
Perhaps something simply became corrupted. Have you tried creating a new drag and drop slide in the same manner?
If it still does not work - build your drag and drop manually rather than use the Question Type.
Go ahead and draw 8 boxes over the answer spaces and tag them as drop targets.
The problem was as you said @Hang! I just had object order such that an unresponsive one covered the responsive elements of the slide. Thank you for your help! You can consider this issue solved