It sounds like the BMS tripped prematurely. This may indicate the cells are not balanced in the pack. Restoring the balance is a pretty slow process with most typical BMS circuits and depends on how the charger works. Balancing will only happen when the pack voltage is at the maximum.
Typical balancing process is to charge the pack for a long time, like at least 24 hours. Some chargers will totally stop charging when they reach full, so no more balancing takes place. For chargers like this, charge to cutoff, then discharge the pack (run the motor) just long enough so the charger will start again and repeat the process many times.
If the pack is new, it may improve with a few cycles as the cells need a little 'break-in'.
If this still doesn't get you close to the expected capacity, then you need to open the pack enough to make voltage measurements of the individual cell groups.