my girl also has this problem then i stop training for a while and then i buy a toll that you feed her drinks water then will vv .
i play with my girl . she loves to play. always feed toll drinks and see the doll vv in the small toilet. after that i give my girl to drink some water and tell her that is your turn to vv. then she like to try sitting on and few times later can vv in her little toilet la. happy.