You may think that having 2 team members and a dragon is a prime reason to gank bot lane, but you're wrong. The general way to play nunu is hope your mid laner carries you and then call all their success your own, even if you had nothing to do with it.
If top lane is feeding make sure to tell your team that the nasus is winning and it's all "thanks" to you. The one think you need to remember is that you should not associate with bot lane at any point in the game, because if a carry has fed multiple kills then you shouldnt be able to kill a support and their adc on half health in a 2 v 3

this is a good second step if being a worthless jungler isnt enough for you