Too much DC load
The Multi RS does not only charge the batteries, it also provides power for the system’s loads.
The battery will only be charged when the power available from the PV panels exceeds the power being drawn by the loads in the system, like lights, fridge, inverter, and so on.
If the system battery monitor is correctly installed and configured you can see how much current is going in (or out) of the battery and the Multi RS will tell you how much current the solar array is generating.
A positive sign alongside the current reading means that current is flowing in to the battery, while a negative sign means that current is being drawn from the battery.