I hear a lot that the handler should train the dog in front of them. Often it is said in the context of training methods - some dogs do best with luring, others can easily be taught to use shaping, etc. Regardless of what you're trying to teach, recognize that - just like people - not all dogs learn the same and you should use the best method for the dog.
But I also was very recently reminded that "the dog in front of you" doesn't stop there. It's not just about the individual, but also about that individual at that specific point in time.
Dogs have off-days, just like humans do. try to remind myself of that, but I think we could all benefit from the occasional reminder - before the dogs in front of us fail to meet high expectations set by the previous training session. But, we are often not conscientious of how the environment of this training session differs from the one before it, not just physically, but also temporally and -probably more importantly - emotionally. Anything that is different this time may affect your dog's performance during a training session: sounds that weren't there last time, being in a different room, whether the dog is tired or hungry (even just compared to last time). Did something really exciting or really scary happen earlier in the day? Is there a meatloaf baking? Does the dog really have to pee (yes, even though they went out 2 seconds ago, Ida)?
All of these are mostly small, and almost entirely invisible things that we may not have consciously identified when we sit down to train the dog in front of us. Even though us humans might not notice them, they can all affect our dog's focus and stress or arousal levels, and thereby impact their performance and its time we started making more of an effort to think about them.