Grouping Dates in Excel Pivot Table

[mskennedy]

I'm working with a pivot table in Excel 2010. I have a date field as my Row Label. I'm trying to Group these dates by Month and Year, but it's not working. When I right-click and select "Group", I should get options to Group By Month, Year, etc. Instead, it just creates a group called "Group1" without giving me the date options. I've used these date groups in other pivot tables, but can't see any difference with this one. Any ideas? thanks.