Drag-and-drop order: even if the answer is correct, Storyline thinks it's incorrect

Hi everybody,

I'm facing a strange problem since yesterday. The solution is probably simple, but I can't figure it out.

I created this slide with a drag-and-drop system: 8 answers, 8 dropping zones (1 answer per zone). Answer 1 has to go on Zone 1, Answer 2 on Zone 2, Answer 3 on Zone 3, etc...
The 8 answers are randomly displaced when you open the slide, and you need to move them to the correct oder.
To help the user, I've created a "temporary dropping zone", but of course it's not possible for the user to click the Submit button as long as there's still an answer on this temporary dropping zone. In this condition, if he clicks the Submit button, he'll see the "unvalid answer" layer.

Now the problem: even though I place the 8 answers in the correct order, Storyline sometimes shows me the "wrong answer" layer and, even more, considers the question as wrongly answered, as shown in the results slide (e.g. it's not just an issue about which layer to show between "wrong answer" and "correct answer": the answer's actually considered wrong while it's correct).
I insist on "sometimes"... because some other times, it works great, which is driving me crazy.

Do you have any idea why?

Thanks a lot.