The ‘Mains’ Lamp:
The ‘Mains’ lamp is on when mains power is applied. After a mains failure of more than 12 minutes, the ‘Mains’ light flashes until mains power is restored and an arm/disarm cycle is completed. The ‘Mains’ lamp also flashes to indicate a low battery condition. When the battery is restored to full charge and an arm/disarm cycle is completed, the ‘Mains’ lamp stops flashing.
So there's three possible causes:
no AC/ adapter faulty,
Panel faulty or not charging the battery,
Faulty battery or leads/connections.
A multimeter can assist you if you have one and of course the ability to use it.
e manno, November 2012