Why are the police becoming exasperated in lamb to the slaughter?

In "Lamb to the Slaughter," the police become exasperated due to their inability to solve the murder of Patrick Maloney, who was killed with a leg of lamb. Their investigation is hindered by the seemingly perfect alibi of his wife, Mary, who cleverly serves the murder weapon as dinner. The detectives' frustration grows as they overlook crucial evidence, highlighting their reliance on assumptions and their underestimation of Mary's cunningness. This irony underscores the themes of deception and the unpredictability of human behavior.