It sounds to me like Sadie is taking life a bit too easy, or, um, er, you are.
She is not as focused on you as she should be. Turning left is very easy as you actually force the dog in a way - turning right she can go wherever she feels like.

Try a check/choke chain and use VERY light corrections at first to get her to obey. It is worth noting the word is correction - this means you are sure the dog knows exactly what you want and when she decides to disobey you "correct" that. Go back a few steps and watch how quick her reactions are to commands she knows well, if there isn't a problem here it is likely that she has no idea and she doesn't understand the right turn.
Some very basic work, concentrating on attention to you and some eye contact work would be useful.
Again, don't start ever correcting until you are 100% sure the dog knows the command - this may of confused her in the first place.