My dog & I are in training for our CDX and until recently his sits have been straight and beautiful. He has started sitting crooked and repetition has not helped fix this. Do you have any input?

My Terrier and I are training for a CDX. He had a beautiful, straight sit until recently. He has started tucking his fanny almost behind my left heel. If I say "sit straight," he corrects his position. Repetition has not helped eliminate the problem. I use only positive training methods and basically no corrections. Repetition solved our previous errors, but not with his curled sit. What am I doing wrong? Thanks a million for your input,

You are right, I have been rewarding him for sitting incorrectly. Boy, do I feel like a dummy! Thanks for bringing my attention to it.