Help with Fillet problems and possible workaround :)

 From:  Michael Gibson
9596.6 In reply to 9596.1 
Hi Andrey, the fillet engine doesn't like planar faces that are fragmented into little coplanar pieces. If you tune up the model to replace the flat side with one single plane that would be better for filleting.

In your case here they're also not quite planar as well. Faces that are at a very shallow angle to each other are also difficult for filleting.

So I'd recommend same as what Barry says above to carve off a little bit of the flat side to make a single plane there. You can either use Edit > Trim + Construct > Planar or also use boolean difference to slice it with a line.

- Michael