When batteries of cells are put together then they may take a few cycles to come to balance.
Lithium cells should have forming cycles done during manufacturing, it's often not clear if batteries do as well as these are rather proprietary/trade secret aspects.
I would try and put the battery through a few cycles that match a cell forming profile. So charge gently and hold at float for 24 hours if possible and then gently discharge to 20%
Try and do that around 5 times if you can. Hopefully you may see the logged capacity creep up a bit.
Gently in this case is probably less than the 15A your charger is capable of, I'd probably do the first couple of cycles at the reduced 4A rate
A good load for a gentle discharge is something like one of the old 50w 12v MR16 house down lights, I have a couple I use for cycling the event batteries.
