A charger that is connected on three phases does not always charge on three phases. These are the usual reasons.
Your car charges on one phase
The onboard charger of the car decides how many phases it uses. Many plug-in hybrids and some electric cars only charge on one phase, at 3.7 kW or 7.4 kW. The charger cannot change that. Check the specifications of your car.
The charger is set up for one phase
During commissioning the installer sets how many phases are connected. If the charger was connected or configured on one phase, it charges on one phase. Your installer can check this and, if the installation allows it, change it by recommissioning the charger.
Solar charging starts on one phase
In the solar modes the charger starts on one phase and only switches to three phases when there is more than about 4 kW of surplus. With less sun, charging stays on one phase. See When does the charger switch from single phase to three phase during solar charging?
Phase imbalance monitoring
In Germany and Austria the charger limits the difference in current between the phases, as required by the grid code. This can reduce the current on some phases. In other countries this function can be switched off by the installer or charge point operator.
Find the reason in the web interface
Open the web interface, go to System and then Diagnostics. The limiting source shows exactly what is limiting the charging power. See My Peblar charger is not charging or charging at low power for an explanation of every limiting source.