I've recently found out about the dungeon crates that you can fish for, well, you guessed it, in the dungeon. I created a large body of water in it, and have been fishing for maybe an hour or so now. I buffed with a crate potion, but the only crates I'm getting are the wooden, iron, and golden. I also keep getting fish from the underground area only, such as armored cavefish. I know that I'm IN the dungeon area, because the music is playing, and the mobs are spawning. So I'm in the dungeon biome, have tons of crate potions, but I can't get anything but underground catches...
The Terraria wiki states this:
"As long as you are below 0 depth and in front of natural Dungeon wall, you are counted as being in the Dungeon. (It is notable that the music changes as well. This is an easy way to tell if you are in the Dungeon.)"
My guess would be that you are not standing in front of a natural dungeon wall.