SSL over Socket hangs on AuthenticateAsClient step

I have a Java socket server using Netty that works fine with SSL/TLS and a Java (Netty) client, but I'm having difficulty getting a Unity C# client to consistently complete the handshake. Two or three times a day it will work, although my copious logging does complain about the server certificate "RemoteCertificateNotAvailable". I have added the remote cert to the Windows trusted certs, and have also added it using my C# code to the Mono store, reading the cert from a file.

I have enabled low level SSL logging on the server, which shows that it hangs after sending the "WRITE: TLSv1 Handshake" bytes, and it does not trigger my RemoteCertificateValidationCallback. I haven't figured out how to enable similar logging on the client. Here is a snippet of my client code, to show how I'm setting the call:

 client = new TcpClient(ipLocalEndPoint);
 client.Client.Connect(ipAddress, port);
 netStream = client.GetStream();
 stream = new SslStream(
             client.GetStream(),
             false,
             new RemoteCertificateValidationCallback(ValidateServerCertificate)
             );
 // snipped lines that install certs from files
 stream.AuthenticateAsClient("tcarrigan.corp.janusresearch.com", null, 
         SslProtocols.Tls, false);

Any useful advice would be greatly appreciated. I'm stumped.