I believe that there is a switch on the microphone to select PTT or VOX. If it is in the VOX position the PTT button won't work. If the radio is set to VOX the PTT won't work. Check all setting for consistency. Otherwise the headset may be defective. Never use VOX - it can get you into trouble. Most VOX headsets have a sensitivity setting or there is one in the radio menu. This combination seems to have neither.