If you have allowed any lead-acid battery (flooded or sealed) to sit around for months to years without recharging it at least every month, the battery's capacity will deteriorate to where it has only a small fraction of its rated A-H capacity. This is due to the formation of Sulphate on the plates. A single recharge does not remove hard Sulphates.
There is only one way to store unused or rarely used lead-acid batteries: keep them fully charged. If fully-charged, the Specific Gravity of the acid is high, which is the only thing that dissolves Sulphates. If the state-of-charge is 75% or less, the S.G is low enough to promote growth of Sulphates. The only thing that moves the S.G. to a range where the Sulphates are dissolved is keeping the battery at a state-of-charge > 95%.
This requires either a continuous "float" charger, or recharging once every two weeks or so. If you put them away discharged, or do not make up for their self-discharge (by floating or periodic recharging), they will self-discharge, reducing the S.G., and promoting Sulphation. Think about this: why does a lead-acid battery in a car that is driven daily last so much more than one in a car that is driven only once a month?
If a lead-acid battery is Sulphated, it is possible to partially restore it. This is done by raising the S.G. and holding it there for a few days. To get the S.G. up to the range where this can happen, charge the battery using a conventional charger. Then use a constant-current power supply set to a low-value (say 2A for a 70AH battery, much less for a small AGM) and leave it applied for up to 72 hours. Under constant-current charge, the battery voltage will climb higher than normal, (~14.8V for a 12V battery). Holding the S.G. at this high value for 1 to 3 days will dissolve what Sulphate can be removed, and the capacity of the battery will be improved (but not as good as new). btw- the name for this is "Equalization".
The infamous "pulse chargers which claim to dissolve Sulphates" floating around on the web are just a pulsed constant-current supply that take weeks to accomplish what a constant-current D.C. supply can do in about 2 days. Caution: Equalization does use up some of the electrolyte in the battery. If the battery has caps, you can add distilled water. If the battery is sealed, there is no way to put back the electrolyte. For this reason, it is better to maintain unused AGMs in a fully-charged state, rather than trying to recover them after years of sitting around discharged.