Question Polybrush not painting with valid shader?

I'm trying to paint a plane with some special shader that I made that IS compatible with Polybrush, but for some reason it works on some meshes and not on others.

On the left, the default Unity plane mesh with the white things painted with Polybrush; on the right a mesh I imported from Blender that I can't paint.

Note: the same mesh from blender with less subdivisions is able to be painted, but as soon as I subdivide it, I can't paint anymore.

What can I do?