Likely that the charger is not taking the cells high enough to get the led's to light. There may be various settings for signalab bms's , but my signalab bms on a pingbattery is set so the led's don't start lighting up till above 3.65v.
Your dodgy cell may have less capacity, so it's the only one getting full enough to light the led. You may want to adjust your charger to a slightly higher voltage to get all the led's to light.
Or,,, it might be just as good a strategy to have the charger slightly undercharge the cells, but still have the bms as a back up to bring down any really high cells. Your call, If you have no cells that remain significantly undercharged each cycle, you might be fine to just charge to 3.4v per cell or so, have less capacity, but potentialy longer cycle life.
THE LIPO RULES. NEVER ABOVE 4.3V NEVER BELOW 2.7V DON'T PUNCTURE
Ideal charging /discharging range for Lipo, 3.65v minimum 4.1v maximum
See battery technology section, FAQ thread at the top of the page for lipo noob info.