The air quality sensor is dirty. Clean the air quality sensor with moistened cotton buds and then dry it with dry cotton buds. Make sure that the air quality sensor is clean and dry.
Pollution sources are larger than the purification capacity. Examples are: when there is heavy polluted outdoor air coming into the room; or when there is new furniture in the room. The new furniture gives out gases for several weeks. The gases and pollutants would be sensed by the air quality sensor.