Tree colliders

Hey all, first post here, kinda new to unity so looking for some help.
I downloaded a terrain pack from the Unity website, and the trees in it don’t have any kind of collider attached. So i dragged the tree i wanted out into the environment, added a mesh collider, created a new prefab and dragged my new tree into it.
Now, if I place the tree+collider by dragging it into the world, the collider works fine.
However, if I go to paint the trees on and select the same tree, the colliders don’t work.

Any tips? I’m stumped, it makes no sense to me.

See pic - (A) is the tree that I manually dragged out and tested, where the collider works.
(B) Is the same tree brought into the terrain painting tool and painted on, where the colliders don’t work.

[1]: http://img24.imageshack.us/img24/2748/unitytreecolliders.png - pic is too big to upload here.

after you made the collider after you dragged out.
you have to make NEW prefab, and drag tree with collider from hierarchy into new prefab again.