Hi xrok1 - what radius are you try to use?

Over here I tried doing the top of the neck, and was able to fillet it at a radius of 0.2 or smaller.

It will be difficult to fillet with any larger radius than that due to the tight bends near the end:



Here I'll try to illustrate the type of thing that causes difficulty. Let's say we have a surface that bends in a tight curve like this:




Now let's say you want to use a fairly large fillet radius (large in proportion to the curvature of the bend) - here represented by a line that I drew:



Look what happens when the filleter will try to maintain this distance as it travels along the tight bend:



See how everything gets bunched together there at the tight bend? That is a situation that causes difficulty in filleting.

If you want to have a larger fillet radius, you will generally need to construct your object to not have bends in it that are near the same size or smaller than the fillet radius that you want to use.

The body has the same issue, to an even greater extent, there is a tight bend in it at this spot:



Seen from the top and zoomed-in:



That makes for a maximum radius of only 0.01 or so. If you make that come to a sharp simple corner instead of a tight tight bend like that, it would make a larger fillet radius possible.

- Michael