Avoid using items like a dragonfly, bread, a small worm, or an apple worm as bait, as these will result in catching only small fish, seaweed, trash, or a sock. Do not use bugs from the character's hair or leftover breakfast, as these yield a broken phone and a necklace.
The key to success is identifying the correct bait. Only the grub obtained from the woodpecker will work to catch the large fish.