Balance, Leona has weaknesses that need to be shored up in the jungle. For one her movement speed is bad. Shes just not that fast moving around.
I actually build boots of swiftness on her after i get the elder lizard. Elder lizard helps shore up her other weakness in the jungle. damage. She just doesn't do alot of sustained damage. Her abilities allow her to do burst but once on cd your doing much less damage. Lizard true damage w/runes and masteries increase the clear time alot. After having lizard and boots of swiftness i usually go sunfire cape. +armor +passive magic aoe damage increases your clear time even more as well as increasing dps on your ganks.
Ganking is important as leona jungle. It is. Level 3 gank is super powerful. However, against folks that ward it renders you useless early because your so goddamn slow they usually run away before u can gank. Therefore i would concentrate on building to clear the jungle faster.
Being prepared for the counter jungle is also very important, make sure to coordinate with your laners to make themselves availible incase you get invaded. 8 times out of 10 an invading jungler will come alone because you are leona ; /. And while you most likely lose to the 1v1 if your teamates our on their toes you almost always win, because they can't get away.
A leona w/lizard/boots of swiftness/sunfire cape is terrifying coming out of the jungle and should kill almost anyone that doesn't have wards set up and even if they do the cc+dive at level 6 is enormous.