Mass Element problem :(

[fajny sz]

Situation:

I've got 2 mass elements: First is a BOX, and second is a FREE FORM with a shape of a curved "extruded" horn. I subtract the horn from the box and I get a box with a horn-shaped opening witch automaticly converts into a FREE FORM.

 

Problem:

I don't know why, but sometimes (depending on the mutual location of the 2 mass elements) I get a FREE FORM with an ability to edit FACEes and to use such functions like SPLITE BY PLANE etc, and sometimes I get a FREE FORM without an ability to edit FACEes and to use SPLITE/TRIM BY PLANE ("The mass element body is invalid"). What does it depend from? Help, anybody!