Minecraft server local difficulty doesn’t change

I’ve been on a server for a while and there have been questions from everyone (including the owner) about why the farms we make are really slow at generating mobs.

I just had a look out of interest on the debug menu and the local difficulty is 1.03 everywhere. Even in the Nether and The End. That’s after 150+ days.

Surely this should fluctuate over time and depending on the area you are in?

Is there a setting or something that would force this value to remain constant everywhere in the Minecraft world?

If so, what is it and can it be removed so that the difficulty can fluctuate as intended again?

We’re running a vanilla Minecraft server on 1.13.1

Thanks