Spotlight illuminating half model

Hi! I have a scene with different types of trees. Ones are my own imported trees and others are from the tree creator. I have a spotlight with a cookie illuminating the trees. The thing is that the trees from the tree creator are ok illuminated (only in the area that the spotlight is pointing and with the holes from the cookie texture). But my own imported trees only have two states: illuminated or not illuminated. And with that I mean that they are not half illuminated by the spotlight with the cookie. I can not see the holes from the cookie on the leaves... I'm kind of newbie and would appreciate your help if someone can explain me why is this happening... Thank you very much