Lamp position matters more than wattage
A strong lamp with the hand in the wrong place cures worse than a small lamp used well.

The thumb is usually the problem
Most lamps cure the four fingers evenly and the thumb poorly, because the thumb sits at the edge of the light field. Cure the thumb separately, rotated toward the bulbs, if your lamp allows it.
Keep layers inside the light path
Very long extensions or a hand placed deep in the lamp can leave the tip of the nail outside the strongest part of the light. Check where the bulbs actually sit before assuming the timer is the issue.
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