RC4 implemetation in C# decryption doesnt working

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C#

private void button1_Click(object sender, EventArgs e)
{
    byte[] key = UnicodeEncoding.Unicode.GetBytes("a");
    byte[] data = UnicodeEncoding.Unicode.GetBytes(richTextBox1.Text);
    RC4(ref data, key);
    richTextBox1.Text = UnicodeEncoding.Unicode.GetString(data);
}

private void button2_Click(object sender, EventArgs e)
{
    byte[] key = UnicodeEncoding.Unicode.GetBytes("a");
    byte[] data = UnicodeEncoding.Unicode.GetBytes(richTextBox1.Text);
    RC4(ref data, key);
    richTextBox1.Text = UnicodeEncoding.Unicode.GetString(data);
}

public void RC4(ref Byte[] bytes, Byte[] key)
{
    Byte[] s = new Byte[256];
    Byte[] k = new Byte[256];
    Byte temp;
    int i, j;

    for (i = 0; i < 256; i++)
    {
        s[i] = (Byte)i;
        k[i] = key[i % key.GetLength(0)];
    }

    j = 0;
    for (i = 0; i < 256; i++)
    {
        j = (j + s[i] + k[i]) % 256;
        temp = s[i];
        s[i] = s[j];
        s[j] = temp;
    }

    i = j = 0;
    for (int x = 0; x < bytes.GetLength(0); x++)
    {
        i = (i + 1) % 256;
        j = (j + s[i]) % 256;
        temp = s[i];
        s[i] = s[j];
        s[j] = temp;
        int t = (s[i] + s[j]) % 256;
        bytes[x] ^= s[t];
    }
}

I am doing this RC4 encryption, i don't know why but when i do the decryption for some reason it doesnt go back to the original text (most of the text does, but some letter doesnt). Do you have any idea why ?