SQL Trigger Update to Uppercase doesn't work using SQLBulkCopy Insertion

I'm using .NET SQLBulkCopy to batch insert data into database. What I'm trying to do is to convert all specified string to uppercase when the data is inserted into the table. Here is my trigger script:

ALTER TRIGGER [dbo].[EmployeeTable_UpperCase]
   ON  [dbo].EmployeeTable
  AFTER INSERT
AS 
BEGIN   

UPDATE EmployeeTable SET 
        [CompanyCode] = UPPER([CompanyCode])
       ,[EmpId] = UPPER([EmpId])
       ,[EmpCode] = UPPER([EmpCode])          
Where Id in (Select Id from inserted)   
END

The script above doesn't work. None of my data is converted to uppercase at all. Does it has anything to do with using SQLBulkCopy in .net to do insertion? How should I make things work in my case?