Simple, some mods add new biomes and you cant catch the new fish in those biomes.

Almost every Fish/Crate drop in Go Fish is dependent on Biome Category rather than direct biomes, so biome mods will/should be compatible. The only outlier are things like the Gilded Blackstone crate, which only drop in the Basalt Detlas.

What fish are you trying to catch, and what biome are you trying to catch them in?

Our issue was no one could ever find a new fish but that was resolved already. Thanks for responding though. We are using https://www.curseforge.com/minecraft/mc-mods/vanilla-biomes and I dont know if we just happened to fish in a vanilla biome and it worked or if its just coincidence.

Good to hear it was resolved, feel free to re-open this issue or create a new one if you encounter it again. Thanks for the report!