Board Thread:Episode Discussions/@comment-25319479-20170920093317/@comment-2112031-20170923130135

True. My main point is that we're never told whether or not Naruto actively trained to use a Rasengan without clones, or whether he simply didn't bother because using clones was easier.

When Kakashi performed the Rasengan, Naruto actually tried to form one witrh one hand as well, only for it to not work, which he then lamented. That doesn't really prove that it was too difficult for him, only that he couldn't do it at that moment.