1) Please check that all your cables are connected properly and securely. Try tightening all cable connections - this includes the USB charger connection into your car cigarette plug, the USB cable connection to the car cigarette plug, and the cable going into the magnetic mount.
2) Another common mistake made while placing the dash cam on to the magnetic holder is not securing it properly with the latch going into the base completely. Thus the dashcam may stick on to the mount, but may not properly power on because the connections are not secure.  
Please refer to the image below and check back with your own camera to see if the connection is completely secure: