Our little shih tzu is 15months old and for the last few months he barks really loudly at guests when they come over. He knows the quiet command. I tell him quiet. He stays quiet for a few seconds then begins to bark again. He will bark on and off until the guests leave. His bark sounds like a stranger invading our home. I allow him to sniff the guests first but I would like him to be calm when guests come over. Any tips.