Datapack works in one world, but not another?

I made a datapack to integrate armour weights from this mod https://modrinth.com/mod/armour-weight with equipment from epic knights. I made a datapack in my testing superflat world, which worked and still works perfectly on that world, but when I exactly copied the datapack to another world it just does not work any more. I tried /reload and relogging but nothing seems to fix it. Why is this happening and how do I fix it?