You must ALWAYS use balance charge with this pack, else you risk a fire. If you have the option to exchange the pack you should do that right away. If not, you might be able to make it work for you.

What is your charge rate? Most balancers have a 0.2A discharge on the higher cell. For packs like yours this is not enough to overcome anything over 0.2A charge rate. Even if you are only charging at 1A, the higher cell is getting 0.8A, while the lower gets the full 1A. If the cells are anywhere close then this will be plenty to balance the pack, but yours does not sound very close.

Try charging the pack at 0.1A or 0.2A. This will take forever, and you will have to check on it often.