First check if your headset is connected firmly, ensuring it is plugged in all the way.  Also ensure the microphone is correctly positioned close to your mouth and not swaying because of loose chin strap - depending on the type of headset you are using. The white marking on the mic boom should also be facing towards your mouth. Test if this works by speaking in a slightly higher than normal voice as you would when riding (not shouting). Another reason is the default microphone sensitivity may be too low (less sensitive) for you. Download the Vertix Velo Microphone Sensitivity Application on your computer (Mac is not supported). Please see Instructions section for more details.  Briefly, you can do this as follows : Connect the Velo control unit to the computer with Velo data and charging cable. Open the application and check the current setting. To set the microphone sensitivity, select microphone sensitivity level followed by the option ‘Write selected setting’. As a general guideline, very high sensitivity is suitable for leisure riders who prefer an open conversation. Normal and high sensitivity is ideal for riders who are riding at speeds of 25 -35km/hr while low sensitivity is appropriate in situations where one is a competitive cyclist and does not want the other cyclists nearby who are shouting to activate their microphone.