• Put the open() as the first thing in openSerial() (actually immediately after the setPortName()).
• Always check the return result of any open()s.
• Remove the open() from writeAll().

bool QSerialPort::setRequestToSend(bool): device not open
bool QSerialPort::setDataTerminalReady(bool): device not open

however, if i try adding either of these, nothing sends through the serial port...

m_serial->open(QSerialPort::ReadWrite);
m_serial->open(QIODevice::ReadWrite);

In which direction does the randomness happen ? From your machine to your device ?
Did you already tried setting requestToSend ? Maybe you need dataTerminalReady.

well, this is the setup:

private:
    Ui::MainWindow *ui;
    QSerialPort *m_serial;

};

void MainWindow::openSeriall(){
    m_serial->setPortName(portName);
    m_serial->setBaudRate(QSerialPort::Baud19200);
    m_serial->setDataBits(QSerialPort::Data8);
    m_serial->setParity(QSerialPort::NoParity);
    m_serial->setFlowControl(QSerialPort::NoFlowControl);
    m_serial->setStopBits(QSerialPort::OneStop);

}


void MainWindow::writeSerial()
{

    QByteArray message;

        message.append(0xFF);


       if (m_serial->open(QIODevice::ReadWrite)) {
           m_serial->write(message);
           m_serial->waitForBytesWritten(-1);
           m_serial->close();

}
}

i cant see anything wrong, i took care of the baud rate, data bits...

maybe no flow control could be causing this? the transmission is successful on a windows pc , strangely enough...

Are there anythings specifically that could cause this to happen?

No.

I'm not an expert, but at least make sure the baud rate etc. parameters are correct under Linux?