mobs dont count for killing if u use a ranged weapon oddly idk why
Is this a pack bug or server bug? That is, does it happen on other servers and/or singleplayer? If it's a pack bug, it should be reported to the pack developers.
This is an issue with Better Questing. This is for a very old version and they are unlikely to continue support. You may want to report it here: https://github.com/GTNewHorizons/NewHorizons/issues
