this is just the nature of AGM batteries
when they go they go
if you look at the performance charts on lead acid v/s AGM batteries you will see that AGM batteries do out perform lead acid but the catch is the fall off rate. Lead Acid falloff rate is a progression where the AGM is more stable on the high end for a longer period but when an AGM battery gets to about 60 to 70 % discharged it drops off like a rock.
They react the same way when they get sulfated or go bad. The voltage may read ok but they don't maintain thier ability to hold a charge well and when loaded up they fail miserably.
Sort of like when your nicad or li ion batteries do in your phones.

Sounds to me like they just need to be replaced (all of them)
If you have 3 that are questionable then you might as well do all of them or you will be back in a short while with the same problem because of that ONE battery you leave in there.

:o)