Why does implementation intentions fail?

The most common reason implementation intentions fails: Writing implementation intentions that are too vague to trigger automatic action. 'When I have free time, I will work on my project' is not an implementation intention — it is a goal intention wearing a trench coat. The power of the format depends entirely on the specificity of the cue. If your 'when' requires you to judge whether the situation applies, you have reintroduced the deliberation that the technique is designed to eliminate. The cue must be unambiguous, the response must be concrete, and the link between them must be so tight that recognizing the cue is sufficient to initiate the behavior.

The fix: Choose one goal you have been failing to act on consistently. Write a standard goal intention first: 'I want to ___.' Now rewrite it as a precise implementation intention using the if-then format: 'When [specific situation/cue], I will [specific action].' The situation must be concrete enough that you would recognize it instantly — a time, a location, a preceding action, an internal state. The action must be specific enough that there is zero ambiguity about what 'doing it' looks like. Write this implementation intention on a card and place it where you will see it tomorrow morning. Execute it for seven days and note what happens.

The underlying principle is straightforward: When X happens I will do Y — this specific format dramatically increases follow-through.