compare a varchar column with a varchar variable does not work

I have two block of SQL statements (I am using sql server 2005):

-- 1

SELECT *
FROM Invoice
WHERE CustomerId = 'ITTEST01' AND StoreId = '01'

-- CustomerId and StoreId have VARCHAR(16) data type.

--2

BEGIN
DECLARE @pCustomerId AS VARCHAR(16)

DECLARE @pStoreId AS VARCHAR(16)

SET @pCustomerId = 'ITTEST01'
SET @pStoreId = '01'

SELECT *
FROM Invoice
WHERE CustomerId = @pCustomerId AND StoreId = @pStoreId
END

The 1st statement returns what I expect, but the 2nd block returns nothing. I am not sure why this happens as the two SQL blocks should return the same result to me? Could someone give me a hint?

Thank you.