Cant import custom collision from 3DS Max to UE4

Hello,

dear UE4 specialists, please give me more rules for creation of complex collision in 3ds max.
Currently, I tried to use Spring object (default from 3ds max) as collision for Stairs. And I got different results for several iteration of increasing polygons number (of collisions).

The first try (success)

The second try (success)

The third try (failed) No collision detected by UE4 engine.

Dear UE4 engineers, could you please give your comments on it?
Why there are so different results in my tests?