Matching drag and drop activity

Because the target boxes are so small, the free answer that's hiding the dropped one doesn't move into an empty slot because it's not on the target. In the built-in method, answers can compete for the targets but in your custom version, the answer choice on the right isn't on the target:

As far as the free answer is concerned, it's on its own starting point, not a target, so there's nothing for it to get out of the way of.

There are probably a few ways of rigging up a solution, including some with JavaScript, but I think the easiest workaround would be to have all of the answer choices stacked together so that none of them are near the targets in the first place. The learner can then drag the top one to a target, revealing the second in the pile, and when that drag that one the third one will show. If they remove or replace one of them, it'll return to its origin, the pile.

Either that or a bucket of triggers, or a single JavaScript trigger to essentially replace and condense the logic the other triggers would use to check which spot is empty and move the now-free answer to it.