On vehicles that originally had a black-plastic Bosch factory diverter valve fitted, it’s really easy to confuse the inlet and outlet of the valve; Bosch valves are usually installed by manufacturers in the opposite orientation to most aftermarket types so try switching the orientation of your GFB BOV around. Trace the hoses that connect to it; the boost pressure from after the turbo should enter the base of the GFB valve, and the dumped air vents through the side outlet(s). Just about the only way a BOV can not vent at all , is if it’s installed backwards. Once you’ve checked that the BOV is oriented correctly, try reducing the spring preload slightly by turning it anticlockwise a couple of turns at a time until it starts to vent.