How to get drop items back to initial state for retry?

I created a freeform question where the learner has to figure out the sequence of a phone conversation. He/she can try as many times as necessary. Yet, if I click "try again", the drop items remain on the targets, instead of being reset to the right side. 

I have attached the question slide. Am I doing something wrong?

Thx for your advise and best regards
Roland

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