Why won't this mesh let itself be weight painted? For some reason, this Mesh won't let itself be weight painted. No matter what I do, I can't really paint more weights on it, specially not in face mask mode which I kinda need.

Does anybody know why?

Sample file: https://a.pomf.cat/wtlzlh.blend
I see two reasons:


Mode is face selection masking: so you need to select faces (A or right click select)
The setting is to subtract and there is no weight to subtract