David.
I think it highly likely that the AGM start battery is not in the same state of charge as the house bank AGMs that have been charged quite differently. 
All house bank batteries should be the same brand, type and age to minimise differences in charging. Further I believe that if you have batteries in series parallel, the in between links should be connected in parallel banks and also with a battery balancer to ensure equal charging to each individual battery.
I'm afraid I agree with Bill, you should replace the complete house bank.
Cheers
Alan
Elyse SM 437