Excell Cell Drop Down Menu randomly lose data when attaching to an email or uploading to Google docs. How to fix?

My employer uses an excel spreadsheet with drop-down menus in some of the cells to keep track of student attendance. When I send the doc via email and upload the doc via Google Drive, sometimes a number of the cell dropdown menus lose their data, getting recorded as though I hadn’t selected an option. When this happens the cells that don’t get recorded are the same in both the email attachment and the uploaded google spreadsheet. What gives and how do I fix it? Yes, I am certain I am saving the doc before uploading or attaching.