In the past two days when I've gone to Castle O to fish up Odontotyrannus(at the ooze pool in H-8), I end up losing my bait without even going into the mini fishing game. Ill cast my line and will get "you didnt catch anything.. you didnt catch anything.. then.. "you giveup".. all without actually seeing the monster. This has happened 4 times since yesterday.

https://ffxiclopedia.fandom.com/wiki/A_Boy%27s_Dream

I was able to complete this quest. Not that after you flag the quest and you trade the willow rod back to aibeche.. you still need to go back and talk to exorche the father again.. then.. go back and talk to aibeche once more, where he'll ask for a giant shell bug and tell you about the mob in CN. From here, trade him a Giant Shell bug, and then he'll tell you about the NM in Castle O. From here you can go fish him up with any rod and any gishing skill. It may take a few casts before you get a bite.

https://ffxiclopedia.fandom.com/wiki/A_Boy%27s_Dream