My characters can't sto falling terrain

Hi, recently I have a very weird error and I don't know how to fix it. I've done my game using Lerpz basics, and once I've finish all the scripts and all my engine I export all my stuff to another project with a terrain done by me. But I experiment a huge problem on this terrain. My characters fall through the terrain, and I don't know what it's the problem, on the previous terrain I had not any problem like this. And my characters are not intersecting the ground, they have they own character controller and the terrain looks like having his collider.

Here you have a screen that shows it:Picture

And the code of grounding:

function IsGrounded () {
    return (collisionFlags & CollisionFlags.CollidedBelow) != 0;

Any idea? Hope you could help me. Lot of thanks

Ok, finally I have solved it in with this option of Unity:

Edit->Project Settings->Physics->collisionmatrix