...besides "beasts", things that are not anthropomorphic (humanlike) will only sometimes wield their weapons, enemies such as Strongroot Tanglebranch only sometimes wields his shield.
boss's weild their weaps 24/7. anyways, you are either having bad luck, or you are trying with allie npc's. dont use hench/hero, they pretty much ruin drop rate. so try to solo with w/n build.