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It could be that at the specific time there was some issue contacting DropBox or the transfer failed for some reason (which should not happen), which caused that.
You should be able to find the files within the Submissions Panel: How to view Submissions
My suggestion is to check if any new submissions (with files) would be sent to your DropBox folder properly or if there is still some issue present.
Sure. I finally find those submissions on jotform.com, but I trusted that they will be there. So I assumed they did'nt submit anything.
Next time I will check first if the number of submissions equals the number of files on dropbox. But It will be fine if this problem is fixed anytime in the future.
All of the files should be recorder into your DropBox folder. However the HTTP / HTTPS protocol through which the transfer is made from JotForm servers to DropBox servers is not a protocol that would allow the sender to see if the files were actually delivered.
Due to this, if any node on the network fails for any reason at all, servers that send the data do not know that the connection broke. In the same time while DropBox servers know that this happened, they might be expecting data to continue with the missing part, or that the files are re-sent, etc, while (due to no clue about anything happening), the expected does not happen.
You can however set up your form with email notification so that you get an email each time the form is submitted.
That way you will have an easy way to see if and when someone submitted the file and if DropBox by some chance happens to not show the same amount of submissions.
On the following link you can see how to create email notification: Setting Up Email Notifications