The problem is that I want to be able to call this send function several
times, but it only works the first time I call it. For example:

Client c(host, port); //remember that the constructor calls the
asynchronous methods
c.send("msg to send");
c.send("msg to send");

The message is actually sent the first time, but the rest of the times
nothing happens. What am I missing? Can you help me?

Obviously, it's hard to guess how your Client::send is implemented, but if it doesn't work as expected, some bug is there.