Attachment is not found after downloading

Hello.

We have a form in Kobo with the obligatory attachment. It works fine, but we have a problem sometimes. When we collect our data, I download all attachments in zip file and unzip it on my computer. Than when I look through the records, for example I need to see a file for this record. When I do a search by id, there is such folder for the attachment, but there is no any file inside. It doesn’t happen for all files, but in very few cases. But still we need these files. So I am trying to understand why it happens. Again the question for adding the attachment is obligatory, so the person can’t submit the form without the attachment. Thank you