Imported FBX causing problems

I am new to unity.
So I have a model imported as FBX and an another one as 3ds,
I created 2 prefabs and put each model in a prefab.
I have a script that changes the cursor on mouse over using the function OnMouseEnter().
The script didn’t work, but when I tried the script attaching to a cube, it changes the cursor.
Why does the imported models cause this problem while the primitive shapes created by unity don’t?

Have a look at the documentation on OnMouseEnter() found here:

You will see that OnMouseEnter() detects the mouse passing over an object which has an associated Collider. When you created a Cube object, Unity gives it a Collider by default. When you import your externally modeled assets, they are not given a default Collider. As such, when you mouse over them and they have no associated collider, the OnMouseEnter callback is simply not invoked.

The quickest solution is to add Collider components to each of your imported models.