SQL SELECT SUM(CASE WHEN [ColumnName] IS NULL, or blank problem

DECLARE @OneStop varchar(50);
DECLARE @CaseManagerLName varchar(50);
DECLARE @FromDate smalldatetime;
DECLARE @ToDate smalldatetime;
DECLARE @ReviewerName varchar(50);
DECLARE @ReviewType varchar(50);
DECLARE @Yes VarChar(1);
DECLARE @No VarChar(1);
DECLARE @NA VarChar(1);
DECLARE @Blank VarChar(1)
SET @OneStop = '%';
SET @CaseManagerLName = '%';
SET @FromDate = '01/01/2000';
SET @ToDate = GetDate();
SET @ReviewerName = '%';
SET @ReviewType = '%';
SET @Yes = 'Y';
SET @No = 'N';
SET @NA = 'N/A';
SET @Blank = '';
 
SELECT OneStop, 'WIA_Adult' AS [Program]
  ,    LEFT(convert(char(10), ReviewDate, 127), 7) as ReviewDate
  ,    CaseManagerLName
  ,    ReviewerName
  ,   'RegSelServ' AS Question
  ,   SUM(CASE WHEN RegSelServ LIKE @Yes THEN 1 ELSE 0 END) AS Yes
  ,   SUM(CASE WHEN RegSelServ LIKE @No THEN 1 ELSE 0 END) AS No
  ,   SUM(CASE WHEN RegSelServ LIKE @NA THEN 1 ELSE 0 END) AS NA
  ,   SUM(CASE WHEN RegSelServ = @Blank THEN 1 ELSE 10 END) AS Blank
  ,   SUM(CASE WHEN RegSelServ IS NULL THEN 1 ELSE 100 END) AS oNULL
 
FROM   WIA_Adult
WHERE   (OneStop LIKE @OneStop)
  AND (CaseManagerLName LIKE @CaseManagerLName) 
  AND (ReviewDate BETWEEN @FromDate AND @ToDate) 
  AND (ReviewerName LIKE @ReviewerName) 
  AND (ReviewType LIKE @ReviewType) 
  AND (RegSelServ IN (@yes, @no) )
GROUP BY ReviewDate, CaseManagerLName, ReviewerName, OneStop