The vent on a range hood has a fan and a filter that draws in greasy and smokey cooking fumes up and into the body of the hood, where it is either ejected out of a pipe through the roof or a filter back out into the room. The filter can become gritty, greasy and grimy and not allow a good airflow. By cleaning the filter with a sponge and soapy water, you may increase the airflow through the vent and get better venting. But if the problem persists, you may need to have a technician check and clean the internal components of the range hood, such as the fan and piping.