Hi Tony - which parts are you trying to fillet, just everything?

Did this model possibly start out at some small scale or something like that and then get scaled up?

Some pieces of it seem to be oddly formed, like this edge here:



When you view it from an angle you can see it's kind of bowed out a ways and it does not adhere too closely to the straight extruded piece:




The edge does not seem to have very good accuracy, it seems to be a pretty significantly sized gap between the edge geometry and the straight extrusion surface. There's a similar thing on the other side of the model as well. That kind of inaccurate edge structure will probably interfere with filleting.

- Michael