Data Validation settings disappear

Good afternoon,

I created a workbook in Excel 2010 which uses data validation settings referencing ranges on different worksheets. When my users open the workbook in Excel 2007, the data validation settings which reference other worksheets are dropped. If I enter the exact same validation ranges directly into Excel 2007 they work fine. Can anyone explain why? My workbook contains a lot of data validation and it would be a royal pain to have to re-enter them all in Excel 2007.

Thanks,
Joe