Data Validation Changes Source on its own

I do not use VBA so I wouldn't expect this to be the cause of the problem. My spreadsheet is locked so no one else has the ability to make changes. I use data validation to create a drop down list using cells on a different sheet in the same workbook. I set the data validation info, save the file, and it's working perfectly but when i send the file to my workmate the data validation source has changed to the same range of cells but on the sheet that the drop down box is on instead of the sheet where the info was selected. When he re-sends me the file and i open the received file it's seen that the source data on the data validation has indeed changed. Where must i be going wrong?