Check power to system
Another common reason for the gates staying open is that the induction loop detector has locked out. To reset the system make sure there are no vehicles or metal objects parked on or near the loop or in the gateway, then switch the isolator off for 30 seconds then back on again, give the gate an open command, wait five minutes and the gates should close.
Finally check the safety beam system shown below. If the lenses are dirty or some vegetation has grown in front of them no matter how small, this will cause the safety system to detect and hold the gates open. Remove any vegetation, clean lenses, stand clear and allow a few minutes for the gates to close.