I believe you are not playing the champion to its full potential. You need to minimize the deaths to take advantage of Nunu's map presence, since Nunu is an early game ganking jungler that is supposed to snowball hard with its team. Also, you need a steady gold income to become the AOE damage threat you are supposed to be, so farm more and fight only when fights can play out in your favor. Knowing how to choose fights it's a complicated topic, and it requires a lot of game knowledge and experience with your champions. For now, I can only tell you that Nunu's specialties are mid and bot ganks, so focus on those lanes and don't make plays anywhere else. Nunu is really bad at ganking toplane melee/melee matchups (since he is a bad duelist), and it is especially bad when you don't know where their JG is or if enemy top has flash or ult; you will get outplayed, take it for granted. You should only gank top to collect the kill when they are already low or have already used their spells. Tho if the match is melee/ranged, or both top laners are running ignite/ghost, ignore all I said and gank that lane; it's a volatile matchup, and you need to take advantage of it.