SQL String Into Stored Procedure

Not sure this is the right forum as I'm not sure quite what the problem is, but I have a feeeling it's the stored procedure that I'm using to replace the SQL string I used previously.

I have a search form used to find records in a (SQL Server 2005) db. The form has a number of textboxes and corresponding checkboxes.

The user types in a value they want to search for (e.g. search for a surname)  and then selects the corresponding checkbox to indicate this. Or they can search for both surname and firstname by typing in the values in the correct textboxes and selecting the checkboxes corressponding to surname and firstname.

The code to make this work looks like this:

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        Dim conn As SqlConnection
        Dim comm As SqlCommand
        Dim param As SqlParameter
        Dim param2 As SqlParameter
        Dim param3 As SqlParameter
        Dim param4 As SqlParameter
        Dim objDataset As DataSet
        Dim objAdapter As SqlDataAdapter

        conn=NewSqlConnection("blah, blah")
        comm = New SqlCommand

        'set properties of comm so it uses conn & recognises which stored proc to execute
        comm.Connection = conn
        comm.CommandText = "SPSearchTest3"
        comm.CommandType = CommandType.StoredProcedure

        'create input parameter, set it's type and value
        param = comm.CreateParameter
        param.ParameterName = "@empid"
        param.Direction = ParameterDirection.Input
        param.Value = txtPatID.Text

        param2 = comm.CreateParameter
        param2.ParameterName = "@LastName"
        param2.Direction = ParameterDirection.Input
        param2.Value = txtSurname.Text

        comm.Parameters.Add(param)
        comm.Parameters.Add(param2)
        conn.Open()
      
        objAdapter = New SqlDataAdapter(comm)
        objDataset = New DataSet
        objAdapter.Fill(objDataset)

        dgrdRegistration.DataSource = objDataset
        dgrdRegistration.DataBind()

        conn.Close()
------------------------------------

While the stored procedure is this:

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    @EmpID int,
    @LastName nvarchar(20)
   
AS

SELECT EmployeeID,
    LastName,
    Firstname,
    BirthDate,
    Address,
    title,
    address

FROM employees

WHERE (DataLength(@EmpID) = 0 OR EmployeeID = @EmpID)
AND (DataLength(@LastName) = 0 OR LastName = @LastName)

------------------------------

This will work if I search using EmployeeID and Surname or only by EmployeeID, but I don't get any results if I search only for Surname, even though I know the record(s) exits in the db and I've spelled it correctly. Can someone point out where I'm going wrong?

(Incidentally if I have a procedure with has only one parameter 'surname' or 'employeeID', it works fine!)

Thanks very much and sorry about the long-winded post.