Attached is a full sized guitar body which got extruded (45 mm thick) from an outline curve.

I'm not able to apply a round fillet of 6 mm radius.

Most of the time I get a clean side surface (without lines) after extrusion and it won't fillet at all.
Some times I get a segmented side surface after extrusion and it fillets fine up to a radius of 5 mm. But not 6 mm where it fillets but the edge remains sharp.

Why is this? Thanks.