Search Sp Not Functioning Correctly ???

can somebody tell me whats wrong with this sp..................when i pass lname only or fname only it does not return any data, but when i passs both lname and fname it returns the correct data.....when i rearrange the structure to where it checks the lname only it works also but fname doesn't .........any help  is greatlly appreciated

 

CREATE PROCEDURE [dbo].[temp_search]
(
  @lname varchar(50) = null,
 @fname varchar(50) = null, 
 @date1 datetime = null,
 @date2 datetime = null
)

/*set @lname = null -- 'aaa'
set @fname =  null --'ddd'
set @date1 = '2/25/2003'
set @date2 = '9/30/2004'
*/

as

set nocount on

 if not @lname is null or @lname = '' and not @fname is null or @fname = ''
begin
 select temp_user_info.*, dateTime1
 FROM temp_user_info
 INNER JOIN temp_seq ON temp_user_info.temp_seq = temp_seq.temp_seq
 WHERE temp_user_info.temp_seq = temp_seq.temp_seq 
 and temp_user_info.lname like @lname
 and temp_user_info.fname like @fname
end
else
  if not @lname is null or @lname = ''
 begin
  select temp_user_info.*, dateTime1
  FROM temp_user_info
  INNER JOIN temp_seq ON cir_temp_user_info.temp_seq = temp_seq.temp_seq
  WHERE temp_user_info.temp_seq = temp_seq.temp_seq  
  and temp_user_info.lname like @lname
 end
 else
   if not @fname is null or @fname = ''
  begin
   select temp_user_info.*, dateTime1
   FROM temp_user_info
   INNER JOIN temp_seq ON temp_user_info.temp_seq = temp_seq.temp_seq
   WHERE temp_user_info.temp_seq = temp_seq.temp_seq  
   and temp_user_info.fname like @fname
  
  end
  else
   if not @date1 = null  and not @date2 = null
   begin
    select cir_temp_user_info.*, dateTime1
    FROM cir_temp_user_info
    INNER JOIN temp_seq ON cir_temp_user_info.temp_seq = temp_seq.temp_seq
    WHERE temp_user_info.temp_seq = temp_seq.temp_seq 
    and dateTime1 between @date1 and  @date2
   end
   else
     if not @date1 = null
    begin
     select temp_user_info.*, dateTime1
     FROM temp_user_info
     INNER JOIN temp_seq ON temp_user_info.temp_seq = cir_temp_seq.temp_seq
     WHERE temp_user_info.temp_seq = temp_seq.temp_seq 
     and dateTime1 like @date1
  
    end
GO