TCP Socket don't want to wait until any data is available. SFLM TCP C++

I have got a problem because i can't synchronize sending and receiving data in two separate threads. In the first one i am sending data by:

this->send(0, std::to_string(nr_of_clients - 1));

in the another one i am receiving data by:

sf::Socket::Status tmp_soc;

    do
    {
        tmp_soc = socket.receive(&data, 100, received);
    } while (tmp_soc == sf::Socket::Status::NotReady);

std::string output_string(data);
std::cout << "Odebrano: " << output_string << std::endl;

If i send data in the way that i am sure that it will reach it destination just before i am trying to receive it, it is all working well. But there are some situations that i can't be sure when i will be able to send and receive data.

Of course, i have tried with:

sf::TcpSocket socket;

sf::SocketSelector selector;
selector.add(socket);

and

if (selector.isReady(socket))
        {
            // this socket is ready, you can receive (or accept if it's a listener)
            socket.receive(...);
        }

but it didn't work.

I would be very thankful if someone could help me in some way, of course i will give best answer mark and so on, so don't worry :) Thank You in advance :)