Are you doing equalizing charges occasionally? (I'll explain this for any newbies who happen to be watching.)
Various cells will have slightly different amounts of leakage. If you do nothing about it, the leaker cells will discharge a bit more than the least leaky and the difference will accumulate. Ordinary charging will stop when the most-charged cells are fully charged, ordinary discharging (with undercharge cut-off) will stop when the least charged cells are near their low point - and being more deeply discharged than the others tends to damage them and increase the leakage. So your pack's capacity drops and your weaest cells get weaker.
An equalizing charge overcharges the battery a bit, gently forcing a little extra current through the fully charged cells to raise the charge state of the undercharged cells. (This tends to make the fully charged cells electrolyze a little extra water into gas, so you don't do it all the time, to avoid running the good cells dry.)
If you have a low cell, the first thing to do is try an equalizing charge, just to see if the "bad cell" just happens to have been a bit leaky and got low. If that works, and you catch it early enough, you've saved the battery. You've also discovered that you need to adjust your charge controller to equalize a bit more than it had been, or see about adding some more generation or reduce your load a bit so your charging system will have enough surplus power to do the equalization on schedule.