Maybelle your first dog? A dog could be disobedient if it willfully refuses to do something, ie. sit 101st time in your living room after 100 consecutive sits. If the dog refuses to sit after the first command then there is a non dog-related problem which could be many things including those of medical, psychological, technical or environmental nature. I would therefore investigate what makes the dog not want to do the exercise anymore. She may have lost interest and motivation in performing the behavior for some reason or she may have a stronger motivation not to perform it.