Why can I encrypt only 16 characters of text?

Works:

string plainText = "1234567890123456";

Doesn't work:

string plainText = "12345678901234561";

Doesn't work:

string plainText = "123456789012345";

Code:

string plainText = "1234567890123456";
byte[] plainTextBytes = Encoding.UTF8.GetBytes(plainText);

byte[] keyBytes = System.Text.Encoding.UTF8.GetBytes("1234567890123456");

byte[] initVectorBytes = System.Text.Encoding.UTF8.GetBytes("1234567890123456");

RijndaelManaged symmetricKey = new RijndaelManaged();
symmetricKey.Mode = CipherMode.CBC;
symmetricKey.Padding = PaddingMode.Zeros;

ICryptoTransform encryptor = symmetricKey.CreateDecryptor(keyBytes, initVectorBytes);

MemoryStream memoryStream = new MemoryStream();

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

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

cryptoStream.FlushFinalBlock();

byte[] cipherTextBytes = memoryStream.ToArray();

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

string cipherText = Convert.ToBase64String(cipherTextBytes);

Console.ReadLine();