Summary: The radii used in FilletEdge can not be so large that they overlap each other and completely consume any surface they are following. This tutorials shows what to do when the one surface is too small for the desired fillet radius.

Download Rhino 3DM file

Original part- note curved floor in the pocket.

Extract floor surface. Offset 4 units (Fillet radius) Pipe the top edge curves (cap = no) to the same radius. Intersect the pipe and the offset.

Pipe the resulting curve(s) at 4.

This is the desired surface - trim to taste with the top edge curve and an isocurve that lies on the floor.

Mirror, join, etc. to complete the solid. Finished polysurface.