Air Admittance Valve

[MartinE]

We have side-by-side two sinks in our kitchen. One has an Inskinkerator and the other a normal drain. There is an air admittance valve fitted to the pipework under the sink. Recently, the normal sink has started to drain very, very slowly.  A friend suggested that the air admittance valve might need replacing.  I replaced it with a FloPlast AV40 Air Admittance Valve  When it is done up tightly on the pipe, the sink won't drain.  If I ease off the screw to let air in, it drains normally.  I thought the valve was supposed to work when tight.  What am I doing wrong?