According to the video (2m 12s) you curl your tongue up. I've never done this. My tongue just flaps. How do you do yours?
Apparently r is the hardest letter as you can't see how it's done.
https://m.youtube.com/watch?v=Ml1VTjLN1Mc&pp=ygUNaG93IHRvIHNheSByIA%3D%3D