I made a character in VRoid.
Imported it into blender as a vrm, then stripped the character of its materials and armature and colliders etc. so I could just get the mesh as a base to make my own character.
Now I'm having trouble putting my own armature onto the character with automatic weight.
Normally I just select the mesh, select the bone and click parent with automate weights. But that doesn't work.
It parents, but there's no weight, the mesh doesn't move when the bone does.
What am I doing wrong?
How is this object any different from one I've made from scratch?

Bone Heat Weighting: failed to find solution for one or more bonesdisplayed in the status bar in orange. There are lots of topics on this forum that give solutions for it. – L0Lock Dec 07 '23 at 17:23