Can't Override Grass Types

I’ve set up a new material to override the original one I have for my landscape. Both materials are identical except that I want to change the grass types in the 2nd instance. However, when I put my new grass types into the overridden material, nothing shows up on the landscape components that it’s assigned to (the textures and all that work fine).

It’s just not loading in the new grass types at all, even if it’s a complete Ctrl+W replica of the original – it only loads them in if I use the same grass types.

Any ideas why this isn’t working? I take it that you can’t override the grass types? Feels like a real shame to me if that’s the case…