Not performed a search by input line

    CREATE DEFINER=`root`@`localhost` PROCEDURE `GetClients`(
clientPIB NVARCHAR(50),
    pageSize INT,
    pageIndex INT)
    BEGIN
    DECLARE startIndex INT;
    SET startIndex = pageSize * pageIndex;
    SELECT Id, FirstName, LastName, MiddleName
    FROM Client 
        WHERE 
        FirstName Like(CASE WHEN clientPIB IS NULL THEN FirstName ELSE '%' + clientPIB + '%' END) OR
        LastName Like(CASE WHEN clientPIB IS NULL THEN LastName ELSE '%' + clientPIB + '%' END) OR
        MiddleName Like(CASE WHEN clientPIB IS NULL THEN MiddleName ELSE '%' + clientPIB + '%' END)
        LIMIT startIndex, pageSize;
    END

Why when I try to perform a search query clients based by client's name, lastname or middlename nothing returned?