A declarer will sometimes have a blind spot in the play of a hand. The winning play is right there but he just can’t see it.
West found the excellent lead of a trump. South won and led the queen of clubs. West won with the ace and continued his good work by leading another trump. South won in hand and was now limited to two spade ruffs in dummy, which left him with only 11 tricks. Another trick would have to come from one of the minor suits. A 4-3 split of seven missing cards is more likely than a 3-3 split of six missing cards, so declarer went after clubs for an extra trick.
He ruffed a spade in dummy and discarded a diamond on the king of clubs. West discarded a spade when South next ruffed a club, so there would be no extra club trick. There was a slim chance that East started with three spades to the king, so South cashed the ace of spades and ruffed a spade. When the king did not drop, the contract was down one. What happened?
South should not have put all his eggs in the club basket. When the clubs split poorly, he still had time to try for a 3-3 diamond split. He should have cashed the king of diamonds, led a diamond to the ace, and ruffed a diamond. Bingo! A spade ruff would get him back to dummy to take a spade discard on the established diamond. Making six!