My soil stack terminates in my upstairs bathroom. The toilet, bath and basin all drain into it about 1m below the top of the stack. I have fitted a Floplast AV110 air admittance valve on top of the stack to ensure that there are no smells escaping into the house. The problem is it doesn’t seem to open! When I flush the toilet, the required air is being sucked into the stack through the bath’s u-bend rather than through the AAV. How do I adjust the AV110 so that it opens when I flush the toilet?? Thanks for any help you can offer!