Hi, I have a barely used Vaillant Ecotec 831. The pressure has dropped a bit so I went to refill it. I last did this several months ago. However, there were no water sounds when I turned the valve on. I unscrewed the filling loop and tried again, expecting to see water gush all over the floor. Nothing. My mains water is fine and I can't find any obvious leaks, so is it the valve itself somehow not working? If so, is there anything I can do please? (Normal plumber I use not answering his phone.) Thanks.