Leo whom I adoted two years ago as a young dog, is totally unphased by a raised voice. I can give him that stern voice and look and he just gives me that "Who? me?" look. Penny (adopted 1 1/2 mos ago) on the other had - I give her a stern NO and she cowers. I think it is a issue of confidence. Leo learned as a small pup that he would always have a place in the "pack" and would always be cared for.
Penny Lane on the other hand, I adopted as an older dog (background unknown) - I always get the feeling that she doesn't realize that she has a permanent home. Although I absolutely love adopting dogs that need special care - there is always the background that affects their behavior. Time and patience will take care of some of it - but they always have that background that we know nothing about that can sometimes affect their behavior. I am not going to stop training Penny however, because I know that not establishing rules will do her more harm than good - once she learns that she is not going to be put out on the street, she definately needs to know that I am alpha