Basically for filleting to be happy you'll either need for pieces to be all one big surface or otherwise to have more separation between areas that are planar.

Right now you've kind of got a half of both ways, you've got most of it as one big surface but then a kind of akward separation kind of right in the middle.

It looks like you started with this shape here:



Then this area here was kind of tacked on separately:



That's left you with some separation between those pieces with edges in these areas here:





If you wanted to do it with all one big surface, then you would want to instead draw in the initial side profile where it was extended some more to the left side more like this:



Then you would cut that with a curve from the top view to slice away material to form the end part instead of kind of gluing it on afterwards:



That kind of approach where you model a larger block and then carve off pieces can tend to make a better end result on things that are supposed to look like they are just one single smooth piece.

If you look at Paolo's example you can see that's how he set it up.

- Michael