Pause before revealing the answer
Give yourself a few seconds to retrieve the word. Recall the example, its first sound or the situation in which you learned it. The attempt itself is useful even when the answer does not arrive.
Diagnose the gap
Ask what was missing. Did you confuse two meanings, forget the pronunciation or recognise the word without being able to produce it? The answer tells you how to practise next.
For a meaning gap, revisit the example. For a production gap, write or say a new sentence. For confusion between similar words, contrast them side by side.
Schedule another encounter
Mark the word difficult and return to it soon, but do not repeat it twenty times in one sitting. Several successful recalls separated by time create a stronger memory than rapid repetition.
Forgetting is feedback, not a verdict on your ability. A useful learning system expects it and adjusts the next review accordingly.