Caliburn.Micro - Value not updating after initial update

Question

I am trying to track the connection status of a server using a TextBlock on my ShellView. The TextBlock updates fine when I launch the program saying "Connected", but I am unable to see me disconnect from the server when I hit the "Disconnect Button". I went ahead and simplified the code to just changing a string value when I hit the button.

Here is my TextBlock on the ShellView.xaml:

<TextBlock Text="{Binding Path=ConnectionString, Mode=OneWay}"></TextBlock>

It is updated on launch of the program using:

public ShellViewModel()
{
  setConnectionString("poop");
}

public void setConnectionString(string str)
{
    _connectionString = str;
}

This method is fired after a button is pressed.

public void Disconnect()
{
   Console.WriteLine("Button Pressed");
   setConnectionString("Button Pressed");
}

I know the button press is registering because the console is showing "Button Pressed". Can somebody tell me why the text on the view isn't updating after the button is pressed?