update not working in gridview

When I hard code the values into the updatecommand then the gridview gets updated, but if I just have the parameters the gridview does not get updated. What could the issue be?

This updates the grid:
UpdateCommand=”Update atable set [afield]=’test’, [bfield]=’test’ where [id]=@id”

This doesn’t:
UpdateCommand=”Update atable set [afield]=@afield, [bfield]=@bfield where [id]=@id”