Whats wrong with Zac?

I really like zac. Most games I play as him I can breeze my way through the jungle, always be sustained and at above half hp, and be able to gank lanes whenever I want, and most of the time sucessfully. However, theres just something that never works out in the late game... I always end up getting good engages, but it isn't enough... Most of the time I lose the game after it hits late game , unless my team just has an insane carry. I cant understand it at all, jungle- wise Zac is absolutely insane with fast clears, good sustain and powerful ganks, but he just doesnt do that well later . I know its not just me, as the low win rates tell something as well.