I came across an interesting article about how to revive what appears to be a "dead" AGM type battery, when it fact it is just too discharged for a battery charger to recognize that it is attached.
The trick it to connect a healthy(fully charged) 12v battery in parallel (positive to positive, etc) with the 'bad' battery and then connect your battery charger to one of the batteries. The battery charger should then start charging the pair, and once the 'bad' battery has enough voltage, you can remove the 'good' battery and continue with the charge.
HotRod.com
The trick it to connect a healthy(fully charged) 12v battery in parallel (positive to positive, etc) with the 'bad' battery and then connect your battery charger to one of the batteries. The battery charger should then start charging the pair, and once the 'bad' battery has enough voltage, you can remove the 'good' battery and continue with the charge.
HotRod.com