onFlowRemoved notification not recevied for NAT service

I'm observing the onFlowRemoved() is not getting called on Flow removal from Switch. NAT has implemented SalFlowListener and does certain clean-up on receiving Flow Removed Notification on specific table(on IDLE_TIME_OUT).

public class NaptFlowRemovedEventHandler implements SalFlowListener {

public void onFlowRemoved(FlowRemoved flowRemoved)

}

Because of this FlowRemoved() is not called, certain SNAT functionality is not working fine.