There is a one way check valve in the water pump that is supposed to prevent any backflow from city water connected sources feeding back through the water pump and filling the fresh water tank. Typically it is caused from plastic bits or other small debris that may have lodged in the one way valve. Sometimes it can be cleared by removing the city water feed temporarily and then using the water pump again a few times to try to clear the debris. Then try city water feed again and monitor your fresh fill status again. The other options are repair/replace the valve in the pump (if possible) or you can install a stand alone check valve in series with the water pump, but that typically requires a little plumbing work etc. as a “bonus ? ”. Good luck.