And 당신 Thought Peter Petrelli Could Fly ... 

I assume that for someone to use a power such as phasing, cyrokenisis, pyrokenisis or in this case, flying, the person using it must have some concentration. It requires less concentration as the person develops their power and uses is it more. This is proved in 'six years ago' when Sylar first used telekenisis to show Chandra. Of course, it is much harder to use a power when distracted. At the start of 'How do you stop an exploding man' D.L. found it hard to phase as he was distracted the the bullet in his gut. Peter found it hard to control the induced radioactivity and, therfore was distracted on proventing himself from going nuclear. Another theorey is that it's impossible to use more than one power at a time.