Hi there,
I've had this problem before and can't remember how I fixed it. Basically there is a weird bug where spreadsheets stored in DropBox don't show up in the recent files when you right click the Excel icon in the Windows taskbar. It's something to do with cloud files I think but there is a way you can get around it. It's really annoying because I always pin my shortcuts down there new files don't show up half the time.
Does anyone know the fix?