Do they by any chance look like these ?


In the first, look at the cell furthest away on the battery nearest the camera - and the cells on the battery upper right.
In the second image it should be obvious - but do look down the whole stack !
These are AGM batteries (aka "dry", VRLA). This is (or rather was) the battery bank for a UPS running at 8kW, two strings of 10 for a nominal voltage of 120V. It was fairly noticeable one morning from the "sulphurous smell" pervading the office when I opened up. The effect you see is (I believe) fairly typical of batteries that have been overcharged/used in a warm environment. They need very little abuse to lose water from them, and being AGM have very very little to start with. Once a cell loses a bit of water, it's internal resistance goes up and so it has a higher cell voltage than the rest of the string (while charging) and has a lower cell voltage than the rest under discharge. This means it will get hotter than the rest, and so develops a feedback cycle with the bad cell getting even more abuse.
If they are not too bad, then you could try adding a few drops of de-ionised water. Don't go overboard, I tried it with the better string of these) and I think I used
a tad too much 
The result is that they promptly tipped some of it overboard.
You are lucky that you can drop a pair off the system fairly easily, though leaving reduced capacity. For us, it needs a complete new set (1 string wouldn't be up to the job), and that's well over a grand in cost
