Apart from being a single child, not having an active job or business of ur dad back home shud be the convincing reason behind visa rejection. As a counselor, one would think that ur parents could settle down to US after coming over here since none is left behind..neither business nor children.. physical properties can be sold....

As you said ur parents already got denied twice...not sure if reapplying again makes the difference......that sucks....
I am not even trying to apply for my parents visa for same reason coz i know they will be denied.