Filtered Gridview Doesn't Want To Update Records

I've got a listbox that displays a list of employee's names.  The employee number is the value stored in the listbox.  I then have a vaccinations gridview that displays all the vaccinations received by the selected employee in the listbox.  For some reason, when I click edit and modify a record and then click update it doesn't want to actually update the record.  It just appears to do a postback and redisplay the record without any changes.  My sqldatasource control is configured as follows:
 SelectCommand="SELECT * FROM [tblVaccinations] WHERE ([EmpNum] = @EmpNum)"
 
@empnum = mylistbox.selectedvalue
 the update command is as follows:  UpdateCommand="UPDATE [tblVaccinations] SET [EmpNum] = @EmpNum, [VacType] = @VacType, [VacIssueDate] = @VacIssueDate, [VacExpDate] = @VacExpDate, [VacInstitution] = @VacInstitution WHERE [VaccinationNum] = @VaccinationNum"