Weird phenomenon with AGM batteries
If the batteries all the same model, and you are doing a complete charge/discharge each day then you've done about 600 cycles.
At this cycle life age I'd expect the batteries are starting to show significant capacity differences. This will mean the terminal voltage during discharge will vary and hence the re-charge be uneven. 

You could extend the battery life by putting active balancing in.
I'd suggest that you should connect 1,3,5,7 and 2,4,6,8 in parallel and then the two strings in series. Then you only need one active balance circuit for the whole group of batteries.   

Even if you put active balancing in it's only looking at the 12 V level and not at individual cells, so variations in capacity/age will still occur. This means you will slightly overcharge some batteries (AGM is very tolerant of this) and undercharge others. Overall it's still better to have the batteries paralleled at the 12 V level to reduce differences as the fully charged terminal voltage will be the same regardless of the battery capacity.