contents of syncd co-edited spreadsheet keep disappearing

I share a spreadsheet on BOX with 3 other editors. For the past week, every time one of us changes the contents of the spreadsheet (via sync'd files on our various computers), the changes show up for us, but the next time someone edits it, the changes have all disappeared. The entire file reverts to the contents from before August 15th. I can see that the updated contents are there in earlier numbered versions, but the most recent version always reverts to the contents of the older one. What is going on?