Check the voltage in each cell with a meter, if all cells are good(they'll probably be at around 3.85 volts per) then it could very well be the balance tab itself. I've had to replace many balance plugs due to a failure. If this is the case it's a cheap easy fix (2.00) requireing a new plug and soldering.

I'd just take the heatshrink off at the end and there is a thin plastic piece that covers the end, take that off as well. From there you can inspect the soldering without completely taking off all the shrink wrap. I need to buy new shrink wrap for mine because I took it all off, but you should be able to remove just the section coving the solder tabs and PCB.

nvrmind guys found the problem. cell 1 was 1.70V & cell 2 was 4.48V!! Im charging the cells individually, then balance them....