> I have fileted the different solids OK, except the smaller free form.
> Do not have any idea why...

In this case it is due to the shape not being entirely smooth, there are 2 spots where it comes to a sort of shallow sharp point.

This is another situation that complicates things for the filleter - when things are completely smooth it generates a simple fillet as a single piece. When things aren't smooth, the filleter has to try to extend and intersect the fillet pieces and doing that at shallow angles tends to be somewhat unreliable.

As you can see, it is not difficult to have situations that won't fillet well. Shapes that don't have unnecessary sharp bits or unnecessary edges in them will tend to work better.

- Michael