I would bet that a part of that is that the owners weren't charging the agm's properly, or with the right charger. A discharged agm won't come back without an agm charger (cheap, at HF) or a parallel lead acid to get the LA charger started.
I think I paid $25 for my AGM/LA charger. Saved at least one $250 red top from the scrap heap for no reason.