Sounds like the BMS is doing its job and shutting down at LVCO. Keep in mind that once the cells get below 3V (12V) they will drop fast as there isn't much power left. Also the cells are not be perfectly balanced meaning they will not ALL hit 2.5V (10V) at exactly the same time. Under normal operating conditions you would want to stop discharging the battery at around 12V and not let the BMS disconnect. The BMS is really more of a last resort safety item like a resettable fuse and should not be used as part of the charge/discharge control system.