Hi Tom, so at a first glance the problem is likely to be related to this edge here (and also on the other side)



The surfaces on either side of that edge are not quite smooth to each other, they meet at a slight crease, you can see it more easily in a full shaded display:




Filleting generally doesn't like that kind of thing where 2 surfaces come fairly close to being smooth but are actually just slightly creased with a shallow 5 to 10 degree difference between them. Building things in a "patch by patch" manner can tend to make that kind of results more easily.

The problem that causes is that when surfaces are totally smooth to one another, the fillet surfaces themselves that are generated on each surface naturally line up with each other. When there are creases between surfaces it means that the fillet pieces will not touch each other directly and so a corner junction patch needs to be generated and it has to figure out how to intersect the fillets with each other and how to build a junction piece. But it becomes difficult to do this when the junction area is very compressed in size and is just a small slivery thing, which is what happens in shallow angled areas like you've got here. When the fillet radius is pretty small, the junction area may be tiny enough that the fillets are close enough to touching so that they glue together ok, and when you increase the radius it gets to be a more difficult slivery area to deal with.

So anyway, that's likely to be the cause of this particular fillet problem.

The solution will probably be to round off that edge first to avoid the "shallow creased meeting" type of situation. I'd probably try to fillet that edge with a large radius or if that doesn't work then cut some area off either side and put in a blend there to make it fully smooth. I'll try those and post the results.

- Michael