Table Filtering

Hi Friends, i have a question table and i want to filter this table using the following stored procedure but i cannot take a resultset ; (there is no error)

Stored Procedure is here:

CREATE PROCEDURE [dbo].[SP$AllQuestion]
@Session nchar(10),
@Class nchar(10),
@Unit nchar(10)
AS
BEGIN
SELECT L.QID, L.CLASS, L.SESSION, L.UNIT, L.CONTENT as QUESTION
FROM tblQUESTION L
WHERE @Session IS NULL OR L.SESSION = @Session)
AND (@Unit IS NULL OR L.UNIT = @Unit
AND (@Class IS NULL OR L.CLASS = @Class)
AND ((@Session IS NULL AND @Unit IS NULL ) OR (L.SESSION = @Session AND L.UNIT = @Unit))
AND ((@Session IS NULL AND @Class IS NULL ) OR (L.SESSION = @Session AND L.CLASS = @Class))
AND ((@Unit IS NULL AND @Class IS NULL ) OR (L.UNIT= @Unit AND L.CLASS = @Class))
AND (L.UNIT = @Unit AND L.SESSION = @Session AND L.CLASS = @Class)
END

i dont know what my fault is ; to your opinion what i have to do?
thanks from now on...

MC