Hi Mark, here is a screenshot of a zoom in on that center area so you can see what Jonah is referring to:



There is a tiny slivered surface on the top there, and because of that the edge structure is not totally clean there, instead of one big long edge there are several little tiny edges in that area.

Little slivery pieces and edges like that really cause a lot of problems for the filleter - it has to try to construct rounded corners in places where multiple edges meet.

Also an additional problem is you've got pieces here that are close to being smooth with one another, but are just a few degrees off from being smooth. This kind of shallow angle between pieces also makes things difficult for the filleter to calculate intersections properly, it is better when things either meet really smoothly or sharply.

For example in this case the outline for the shape from the top looks like this:



The end tangent direction of the curve is defined by the last 2 control points, circled above. In this case, these are not horizontal with one another, the line between them forms a slight angle, so the shape you construct from them will also have a slight angle in it and mirroring such an object forms a slight shallow crease at the mirror juncture.

I've attached here a version test8_new.zip, where I have reconstructed this object after using Transform/Align to true up those 2 end points with each other to make them meet smoothly. This version should now fillet ok.

One quick note on filleting - sometimes a nice shortcut for filleting is to select faces to fillet instead of edges (all the edges that belong to the face will then be filleted). Like to fillet all the top edges in this case you can select the 2 top faces instead of all the little edges.

- Michael