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If you had used the search facilities here you would have found this thread https://care.king.com/en/candy-crush-saga/candy-crush-saga-level-1351 which fully explains how the level works. In short the keys will always drop on specific turns and only on those turns. If you fail to make a space directly under one of the key entry points on that move the drop will not take place and you will therefore be short of keys at the end.