The airflow through the machine causes the water movement. When your machine is on for a couple of hours, an ever-growing foam layer (which is the dirt trapper) will form on the water surface the longer the machine runs. This is due to the release of oil-, fragrance- and water molecules into the air and in ratio, becoming less compared to the trapper component (foam) which stays behind. The foam layer acts like a buffer suppressing the movement of the water by sticking to the side of the bowl. The thicker the foam the less movement you will have. Due to difference in volatility of certain oils this could happen sooner with some, than with others. Please shake your fragrance bottle well before adding 5-10 ml to the water in the bowl, as inconsistency of the components can cause excess foam. Chemicals in the water and air can also be a cause. It is better to use filter water if your tap water is not of a good quality. The solution for excessive foam is to clean the bowl, add fresh water and fragrance and switch on.