I’m a bit confused about issues related to wind pressure and air volume

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Our company has two fluidized beds, each equipped with an exhaust fan that draws air in order to keep the material in the bed suspended. Initially, due to the high power of these fans, it was sufficient to open the valves only to 1/3 of their capacity; this led to significant powder leakage from the fluidized beds (as the material leaked through the small holes in the bed structure). Later, a frequency converter was added to the fan of one of the fluidized beds, which reduced its speed and allowed the valves to be opened to a greater degree. As a result, the powder leakage decreased significantly, to about 1/6 of the original level. However, the fluidized bed without the frequency converter continued to experience heavy powder leakage (even after switching to an exhaust fan with lower pressure and airflow, since the motor power remained unchanged). I would like to ask, what is the reason for this? Why does adding an inverter help to **reduce** the amount of material that falls? How do the wind pressure difference and wind speed inside the bed frame change before and after installing the inverter?