Have found that when you arm the system (either in delay mode or instant mode) all sensors have to be quiet or made. If you are in delay mode, you can then open and close door sensors, and one minute later the system will arm. If any of the sensors are open when you try to arm, in either mode, you'll get the symptoms you described. Seems a little user-unfriendly to me - I have to close the garage door before I start leaving the garage...