Cells draining unbalanced out of nowhere

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My mod that I built myself has been working fine with a maxamps 2250 for 2 months......The last week it is draining the cells especially cell 3 out of wack.....when I charge it it will finally balance out but takes way longer than it used to.....cell 3 will only get to 4.17 So I tried my fully max 900 and changed watt hours like I was suppose to.....It was balanced till I used it and charged it again and now cell 3 will not get to 4.20 in this different battery to Any Ideas John or anybody?