Problem with Receiving Bluetooth Data?

I have an App that connects to bluetooth with another bluetooth device but when receiving the data the following happens:

I receive the data cut for example:

Recibo: Primer Dato

Pero Logro Recibir: imer Dato

In other words, missing characters are missing and the String is cut

The problem has to do with this:

 bluetoothIn = new Handler(){
        public void handleMessage(android.os.Message msg) {
            if (msg.what == handlerState) {
                String readMessage = (String) msg.obj;
            //    Toast.makeText(MainActivity.this, "Dato Recibido Entero: " + readMessage, Toast.LENGTH_SHORT).show();
                DataStringIN.append(readMessage);

                int endOfLineIndex = DataStringIN.indexOf("#");

                if (endOfLineIndex > 0) {
                    String dataInPrint = DataStringIN.substring(0, endOfLineIndex);
                  Toast.makeText(MainActivity.this, "Dato Recibido: " +dataInPrint, Toast.LENGTH_SHORT).show();
                    DataStringIN.delete(0, DataStringIN.length());
                }
            }
        }

    };

I think it may be the way that the String is saved and that I use StringBuilder called DataStringIN to save the received data and the way the loop is used for

and the Run method that reads the data can also be the problem:

  public void run()
    {
        byte[] buffer = new byte[256];
        int bytes;

        while (true) {
            // Se mantiene en modo escucha para determinar el ingreso de datos
            try {
                bytes = mmInStream.read(buffer);
                String readMessage = new String(buffer, 0, bytes);
                // Envia los datos obtenidos hacia el evento via handler
                bluetoothIn.obtainMessage(handlerState, bytes, -1, readMessage).sendToTarget();
            } catch (IOException e) {
                break;
            }
        }
    }

Here I have my suspicions that it may be a string or Array [] that is being used badly