One of the most common frustrations I hear from rally and obedience teams is, "But he knows this." And usually, they're right. The dog can sit. The dog can heel. The dog can pivot cleanly and perform beautifully in the space where the behavior was originally taught.
The problem shows up when something changes.
You move from the living room to the kitchen. You go from the basement to the garage. You step outside. Suddenly, it looks like the dog has never heard the cue before. This is almost always a generalization problem.