Can be anything in game from missing cs to always getting zoned out to just a champion you despise. I figured I'd mention mine.
I know jungling is a painful role to do and you need to gank whenever possible but for crying out loud, don't go after the lane which has a 10 HP enemy that won't be able to escape on their own with your ally going for the kill by one simply auto attack. Yet the jungler for whatever reason, blows flash or goes through a wall to go swipe and steal the kill for no good reason. Seriously, why? What's the point of doing that when the kill was guaranteed? If the enemy was say Hecarim and he bolted away, fair enough so you could give chase but someone who has no easy escape/no summoners and is about to die? No... no point in that.