SQL Query taking long time
===

I'm trying to execute stored procedure on SQL Server 2008 from C# application. It takes a lot of time and my application "not responding". My stored procedure:

    ALTER PROCEDURE [dbo].[PROC066]
    @date_1 date,
    @date_2 date
    AS
    SELECT
    RTRIM(tt1.[Org]) as 'Org', 
    RTRIM(tt1.[CustomerHQ]) as 'Customer',
    RTRIM(tt1.[Customer]) as 'ShipTo', 
    RTRIM(tt1.[Date]) as 'Date', 
    RTRIM(tt2.Clean) as 'Clean', 
    RTRIM(tt1.[PO number]) as 'PONumber',
    RTRIM(tt1.[GCAS]) as 'GCAS',
    'Description' = (SELECT DISTINCT [Description] FROM tDeploymentDB WHERE tDeploymentDB.[Item Code] = tt1.[GCAS]),
    RTRIM(tt1.NQTY) as 'NQTY',
    RTRIM(tt1.[Dummy]) as 'Dummy', 
    RTRIM(tt1.[MOQ]) as 'MOQ', 
    RTRIM(tt1.[Inactive]) as 'Inactive', 
    RTRIM(tt1.[Allocation]) as 'Allocation', 
    RTRIM(tt1.[MultiSector]) as 'MultiSector',
    RTRIM(tt2.Pallet_Checked) as 'FullPallets',
    RTRIM(tt2.FullTruck_Checked) as 'Full_Truck',
    FROM tCleanOrderTracking_prod as tt1, tCleanOrderTracking_HDR_prod as tt2
    WHERE 
    SUBSTRING(RTRIM(tt1.[PO number]), 6, LEN(RTRIM(tt1.[PO number])) - 5) = 
    RTRIM(tt2.CustomerOrderNumber) AND 
    tt1.[Date] = @date_1 AND tt1.[Date] <= @date_2
    ORDER BY Org, [PO Number]

If I'm trying to execute this procedure on SQL Server Management Studio - it takes 5-7 sec. But through C# I can't to execute this query. I was trying to delete this row in query  

    tt1.[Date] = @date_1 AND tt1.[Date] <= @date_2

After that it works fine.. my application takes 5 sec for executing. Also I was trying to rewrite this row

    tt1.[Date] BETWEEN @date_1 AND @date_2

No result! My main table in query tCleanOrderTracking_prod have about 40500 records. What can I try else? What I'm doing wrong?