Can't re-enter the game room

When I try to re-enter the game room after staying in the lobby for a while, nothing happens and I can't get in the game room. First entering and re-entering use the same Unity source code.
Another case... I put the game on hold by pressing Home button(Android). I keep it on hold for some time.
In this case, mostly the game comes back to the lobby without any human actions.
In this case, I can't re-enter the game room either. I have to quit the game and start it again to get in.

One more question. I sometimes get this error at the beginning, saying...
Connect() to 'ns.exitgames.com' failed: System.Net.Sockets.SocketException: No such host is known...

I haven't found any clear explanations about this. Can anyone enlighten me?

Thanks for any help.