Sounds like your charger is stopping at a specific mah point, but some packs will take a bit more than the advertised capacity, so if your charger can be adjusted, I'd suggest setting it for a little more(maybe an extra 1000mah above the rated capacity of the pack), & don't worry, the charger should still stop the charge at the right time(as long as it's set in a proper CC/CV LiPo charge mode)....