Abi, I would be concerned if a dog was not excited about training, especially if I had started that dog from a pup.
My dog Lance, who I adopted from a shelter after he had been placed with a family for a year and then returned when they had a baby, is a fairly classic case of a dog being crushed through the use of harsh techniques on a very “soft” dog. He knew “sit” very well and “come” reasonably well, but there was absolutely no joy in it. When I would work with him, even though I used totally positive techniques due to his background and temperament, you could tell that anything that resembled “training” was almost a punishment for him. He would completely shut down. He would do whatever he had to do, but only because he was afraid of pain if he did not. After I had him for about a month, he started to realize that I had never hurt him. One day, he refused to sit and you could tell from his expression that he was testing the waters to see what would happen. I have never been so happy to have a dog disobey, but that was a HUGE step for him! I calmly repeated the command and helped him into a sit. From that point on, he started improving. He still does not have the joy and excitement for training that my other dogs have had (and likely he never will), but he also doesn’t dread it. And he definitely has a strong trust in me now that I am not going to hurt him, so he is open to learning new things.
Hopefully you will never have to deal with a dog who is that shut down, but lack of enthusiasm for training is certainly a warning sign for me that the training is becoming more negative than positive and that I need to rethink my approach or expectations. It will be interesting to see what others look for!