Since Kakashi couldn't add his lightning element to the Rasengan, could it be because it would distort it?
I guess I was thinking back during Naruto's training of learning the Rasengan & was given balloons to practice with. In a sense, if the same method was given when trying to apply an element, I could understand why lightning didn't work.
Also, though I could be wrong, the Chidori required a certain shape & nature manipulation. So perhaps there are limits as to what elements you can use on a technique.
I don't know, it's jumbled up in my head so kind of hard to explain it in words. xD