Your knight mates a king boxed in by its own pieces
The enemy king is hemmed in by its own pieces with no escape. A knight check it cannot block or capture is mate. You miss the pattern, or miss the queen sacrifice that sets it up.
vs blitz game: the knight check was mate on the spot, the king walled in by its own rook and pawns. Played a slow move and the king slipped away.
vs puzzle: a queen check drives the king to the corner, a queen sacrifice blocks the last flight square, the knight mates.