The last I heard, once an AGM battery falls below a certain charge level, it requires a special charging procedure and charger. If you have a Battery Warehouse in your local give them a call for the procedure.
My 2 cents .......

 

I killed AGM batteries about four times in two years in my Pro Street 'stang that I drove only VERY occasionally. Yeah, once they get drained they don't ever like coming back. The guy at Auto Zone said he'd not give me another one until I fixed the problem. (I had forgotten switches on those times it died, my bad)

 

I do a manual desulphication (sorry I dont know how to spell that)
Charge at15 amps or there abouts. The charging time will progressively get longer by the 10th time or so.
1st time it will charge in 10 minutes. Because that's the diminished capacity.

I believe the cteck chargers have this algorithm built in, but very low amps.
I use high power charger, so I dont know if the ctek can bring a dead batt back.

 

The CTEK US7002 model has a specific "recond" mode which is designed to bring back severely discharged batteries. Like other CTEKs it desulphonates with pulse charges averaging about 1.5 Amps. The reconditioning process takes about twelve hours to recondition and recharge a 90 Ahr battery, and about twenty hours for a 120 Ahr battery.