A dog sits perfectly in the kitchen and appears to have never heard the word in a park. This is not defiance, and it is not a failure of memory.
What the dog actually learned
Learning attaches to whatever was reliably present when the reward arrived. In a kitchen that includes the floor surface, the lighting, the owner's posture, the smell of the room and the sound of the fridge.
The word is one element in that bundle rather than the definition of the behaviour. The dog has learned a situation, not a term.
Change enough of the bundle and the cue no longer resembles the thing that was trained. From the dog's position it is a new problem with a familiar noise in it.
The elements owners never notice
Body position does far more work than most people realise. Many dogs are responding to a slight forward lean, a hand at waist height, or the owner standing rather than seated.
Testing this is simple. Give a well-known cue while sitting on the floor, or with the back turned, and the response frequently disappears entirely.
Treat location is another hidden element. A dog that has only ever been rewarded from a pocket often responds to the pocket rather than to anything said.
How generalisation is actually built
The behaviour has to be retrained, briefly, in each meaningfully different context. This is not repetition of the same work but deliberate variation.
Useful dimensions to vary are location, surface, the owner's posture and distance, the presence of other people, background noise, and the level of distraction.
Each new context takes fewer repetitions than the last. After a handful of settings most dogs begin responding in a novel one immediately, which is the point at which the cue has genuinely generalised.
Why difficulty should move one step at a time
A common error is to change several dimensions at once — a new location that is also noisy, also outdoors and also full of other dogs.
The behaviour fails, the dog is repeatedly asked and repeatedly does not comply, and the cue is weakened by being said into no response.
Raising one dimension while lowering another keeps the success rate high. Adding distraction usually means reducing distance and duration at the same time.
The word survives even when the behaviour does not
A cue that has been repeated into silence many times acquires a new meaning: this word sometimes precedes nothing.
Rebuilding it is harder than teaching a fresh word, which is why trainers often simply change the cue rather than repair a poisoned one.
The underlying discipline is to say a cue only when the dog is likely to succeed, and to make the situation easier rather than repeating the word louder.