Minecraft Version: 1.16.4
Forge Version: 35.1.13
Mod Version: cagedmobs-1.16.4-1.2.6

When harvesting Wither Skeletons, the "Wither skeleton skull" item in never dropped, only bones and coal.
When wither skeletons are killed normally in the world, they do drop the skulls

Did you upgrade your cage with the lightning upgrade?

Ah, that has started the Skull drop. I thought that wither could drop heads normally though, not just from a charged creeper explosion.

Problem solved for now though. Thanks!