Why were Hiro’s powers so sporadic?

The show Heroes was originally written as a one season series, so a lot of the stories and characters were were not thought out in a way that would lead to more development. The second season of Heroes was affected by the writers' strike. Story lines were dropped in the middle of development and writers were changed. It caused a lot of inconsistent plots and resolutions. It was shown in Season One that his own confidence in his abilities seemed to affect them. When he was in Vegas he seemed to lose his powers, because he had lost confidence in his ability due to the killing of Charlie the waitress and his inability to stop or change the outcome. He later seems to master or at least have control over his powers, as well as the ability to use a katana by the end of Season One.

It’s also shown in Season One that in the future he masters the English language and also has gone years without letting anyone know he could time-travel as well as teleport. In later seasons Hiro seems to lose his powers for reasons that aren’t always given. When thrown back in time to Japan he can’t get back to his own time. Later on he is reduced to the mental state of a child and given a brain tumor which is killing him.

The real reason this happens is inconsistent writing. Also Hiro and Peter both had extremely strong powers. They both could have potentially solved any conflict with their powers. So the writers had to start writing weaknesses that they didn't have before. Peter was only able to keep one power at a time. Hiro's ability to use his powers become more sporadic.

The canon reason as far as Season One goes is that Hiro's power is one of the greater ones and much harder to control properly. The easiest for him to do is teleport and stop time. Using time travel effectively seems to be the hardest. Future Hiro has an extensive 3-D time chart that he has used to figure out exactly how to change the future for the better. So even with mastery of his powers he is struggling to try to change time. The only way he can do it is to time stop Peter on the subway and give him a vague message "Save the Cheerleader, save the world". Also it seems that the further back in time Hiro goes the longer it takes him to use his powers again, almost like they need to recharge.

The argument can be made is that some people who have powers just have better control over them than others.

Looking at Michah, he seemed to have his power under control at a very young age whereas his mother could only tap into her power while her split personality is active.

Peter couldn't control his power until Claude forced him into controlling it by tossing him off a building. While his brother, Nathan, seemed like he was flying forever once he learned how to control it in a short time.

Heroes tries a very human approach to the powers used and I believe this helps portray that by showing people that just have better control over their abilities than others just like some people are just smarter than others and other skills of the sort.

It's the writing.

But, if you were to put in the context of the show, it just breaks down to the extent of the control of your powers.

I couldn't comment otherwise on this entry. That is the only reason I entered it as an answer. Am I missing where I can comment or is there some other reason that I have not discovered yet.

Also, before Peter could control his powers, he would randomly grab/copy the power of another 'special.' Once he ran into Claude, he figured out how to 'access' each of the powers he had 'filed away'/absorbed.