Can't reference other worksheet for Data Validation in Excel - Plz Help!?

I created a workbook with several worksheets. I created one worksheet called "Validation". On that sheet, I created lists that I want to use on another worksheet. I named those lists properly in the "Name Box" (ie. ValidCauses, ValidSupervisors, etc.). I then created a Data Validation list on another sheet. I selected "List" and typed "=ValidCauses", etc (without the quotes). Everything worked great. I copied this as a template to another computer for others to start using it. It worked great for a few days. Today I find that the list no longer drops down and when I try to link to my list on "Validation" I get an error, "You may not use references to other worksheets..." I only get this problem in that work sheet. I can link just fine on other worksheets. My original file still works fine. I'm trying to figure out what they did to stop the Data Validation on that one worksheet. I know I can fix it by copying the data, but I'd like to figure out what caused it.