CockneySteve is on the right track, ok a LiPo cell is not the same as LiIo cell but some of the characterics are the same. To get the full power of the battery all the cells must be balanced, any out of balance in one cell will reduce the capacity available from the pack. I can well believe that the cells that have failed will be out of balance. It should also be noted that with this out of balance you are still drawing the same current and guess what the pack temperature goes into orbit and the packs swell like a balloon. As all users of Lithuim batteries know balance is critical to get the capacity and also long life. Equally you can still charge an out of balance pack without any issues but you will still be down on capacity and risk of a failure.