User blog comment:IamJakuhoRaikoben/Chapter 259: Porlyusica, Review./@comment-4449950-20111113111609/@comment-76.161.172.226-20111118191151

It's simple. After having used the Lightning/Fire mode for multiple attacks, Natsu got the hang of it well enough to try and duplicate it without having Laxus's magic forcing the modification. He can't duplicate the power up from Jellal, because it was just a mass of fire magic that didn't do anything to change his magic, it just provided more power.