This is a video of Tobin Eade doing 5 neutral grip chins with 20kg. It was the final set of his chins for the day and he still had a rep or two in him if he had to grind them out. He is also carrying a cork in the forearm so we took it a bit easier than he would have normally pushed it. At 15, and only having trained in the gym for just over 6 months, it is a really good effort.
When he started he could get 6 chins at body weight so Tobin already had a decent base. To take that and basically add 20kg to it in this short amount of time is good going. We have done it through a variety of different rep ranges, we use multiple grips for the chin ups, use all different types of tempo's to manipulate the time under tension, and we continue to work on the upper back and strength of the external rotators through many different assistance type movements. Whilst these help chin up performance, they are also helping his bench press increase quite quickly too.
Know what you need to work on and use the appropriate progressions and training variables and you will be able to progress.