As a rule of thumb battery maintainers are used on cars that are driven once a week or less. They don't have the amperage to re-charge a battery, simply maintain a healthy battery that's parked for long durations.
AGM batteries can be fussy with age, like early cell phone batteries. They will not take a full charge even if the car is driven 16mi a day.
I tried this on my 5 yr old Odyssey AGM battery and if worked.
Fully drain and recharge with a real battery charger that 30amps or grater. This was suggested to me by an engineer who works of Odyssey. Her explanation was under cretin conditions, such as short high amperage charge, then plugging it to the maintainer will sulfate the battery over time.
I drained and charged it twice, so far so good, saved me from buying a new battery.