Fit Filleting Fails

 From:  Lara (MALA)
11905.1 
Hello,
In my last project, I gained some experience in rounding lines at their points of contact.

But here, I can't round any of the corner points with Fit Fillet.
Why isn't it working, and is there a workaround?

I would really appreciate some help...

Cheers, Mala