Assignments and Images Deleted

Our team is new to InCopy. We had successfully done a couple rounds of editing and sent the proof to our copyeditor, who is on a PC. She was having problems accessing the file with all of its fonts (even though everything had been working fine before). When our designer went to fix it, he found that all his assignments appeared to have been deleted as well as all of his images. He had to go searching for images, and in one case, recreate the image.

We are trying to figure out what might have caused this to happen. No one intentionally deleted anything. 

We keep our files on dropbox, however we've been careful to always check files in when done with them. The files have not been moved around, but kept in this one location. 

We asked another team for help understanding why this happened. They claimed that they have never had the problem and maybe it's because we have chosen to have our process be to open the InDesign file in InCopy and open assignments from there, rather than opening assignments one-by-one from finder. However, we have been working succesfully with that method until this moment.

Any ideas what might have caused this?