decryption Method doesn't work properly in c#?

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C#

WinForms

encryption

text

when I decrypt the encrypted text it doesn't decrypt the first few words of the document.
My Encryption method is :

public string EncryptString(string plainText,string key)
        {
            RijndaelManaged rijndaelCipher = new RijndaelManaged();

            rijndaelCipher.Key =CreateKey(key);            

            MemoryStream memoryStream = new MemoryStream();

            ICryptoTransform rijndaelEncryptor = rijndaelCipher.CreateEncryptor();

            CryptoStream cryptoStream = new CryptoStream(memoryStream, rijndaelEncryptor, CryptoStreamMode.Write);

            byte[] plainBytes = Encoding.ASCII.GetBytes(plainText);

            cryptoStream.Write(plainBytes, 0, plainBytes.Length);

            cryptoStream.FlushFinalBlock();

            byte[] cipherBytes = memoryStream.ToArray();

            memoryStream.Close();
            cryptoStream.Close();

            string cipherText = Convert.ToBase64String(cipherBytes, 0, cipherBytes.Length);

            return cipherText;
        }
private byte[] CreateKey(string password)
        {
            var salt = new byte[] { 1, 2, 23, 234, 37, 48, 134, 63, 248, 4 };

            const int Iterations = 9872;
            var rfc2898DeriveBytes = new Rfc2898DeriveBytes(password, salt, Iterations);            
                return rfc2898DeriveBytes.GetBytes(32);
            
        }

Decryption Method is :

public string DecryptString(string cipherText,string key)
       {
           RijndaelManaged rijndaelCipher = new RijndaelManaged();

           rijndaelCipher.Key =CreateKey(key);

           MemoryStream memoryStream = new MemoryStream();

           ICryptoTransform rijndaelDecryptor = rijndaelCipher.CreateDecryptor();

           CryptoStream cryptoStream = new CryptoStream(memoryStream, rijndaelDecryptor, CryptoStreamMode.Write);
           string plainText = String.Empty;

           try
           {
               byte[] cipherBytes = Convert.FromBase64String(cipherText);
               cryptoStream.Write(cipherBytes, 0, cipherBytes.Length);

               cryptoStream.FlushFinalBlock();

               byte[] plainBytes = memoryStream.ToArray();

               plainText = Encoding.ASCII.GetString(plainBytes, 0, plainBytes.Length);
           }
           finally
           {
               memoryStream.Close();
               cryptoStream.Close();
           }

           return plainText;
       }

My key is : "ghb" (which I provide when decryption method is called)..

Methods call :

textBox1.Text = BCF.EncryptString(textBox1.Text,"ghb");//encryption time 
textBox1.Text = BCF.DecryptString(textBox1.Text,"ghb");//decryption time

How to solved this method ?

Regards
Jayanta.