Hi Leonard, just to clarify what Burr was saying, the spots in your model that have some problems are these areas at the ends here:



If you zoom in closely to those spots, you can see that instead of just being a simple structure in those spots there are some tiny little slivery bits in there:




Those are what are messing up the fillet, also in the bottom of one of those areas is a little gap which meant the object didn't quite get all joined up into a full solid.

I've attached a fixed up version of your model - to fix it I drew in some lines at those ends, and moved the line inwards slightly and then just trimmed off those little slivery parts to make clean ends instead. Then I used Construct > Planar to fill in plane caps in those spots, and also I noticed a couple of edges that were fragmented into more than one segment instead of being a long edge, I used the Merge command to get those glued together into long edges.

This version should now fillet ok.

- Michael