Greetings for all!

Sorry, I saw many related topics speaking about this, but none of those helped me, so I decided to ask here.
I am studying UDK from about 3 months ago, and I am facing problems to export my own trees to UDK. For example, I tryed several times to do and do again my colisions! and none of those attempts really satisfied me!

The first 3 trees that I made worked perfectly! I don't know why but.. My fourth , fifth and sixth trees were initially not having colision. After several trials, the fifth and the fourth were almost fixxed (more or less), but the sixthy I don't imagine why, it is totally bugged.

when I put the object on the scene, it is tiny, and no problems, it blocks my passage, but when I increase its size, the problem begins.

Can you guys help me? I could show images with collision, and you point me a defect to fix... I imagine that the size of collisions do not increase proportionally as the size of tree.. but I am confused, what should I do?

How to solve that problem?