I tried to make fillets to corners. Why MoI doesn't do the fillet to the corners?

Hi JL, it's probably because these surfaces have a type of malformed shaping:



Some corners of those surfaces are not distinct, they come to an "anti-tangent" type arrangement here:



Also another thing that's not good for filleting is a kind of tight bend in the surface here:



Getting a good shape on the ends of this type of shape tends to be more difficult than you might initially think. It's better to make an extended capping surface which will then get trimmed rather than trying to fill in those ends by building surfaces directly to the edges, see here for some strategies:
http://moi3d.com/forum/index.php?webtag=MOI&msg=1002.2
http://moi3d.com/forum/index.php?webtag=MOI&msg=3137.1
http://moi3d.com/forum/index.php?webtag=MOI&msg=6134.1

- Michael

Hi JL, please see attached 3DM file for a strategy that will work better for filleting.

The steps I used were like this - I made a profile curve like this:



Then I did a sweep using the longer neighboring edge as the rail curve:



That makes a surface like this which has much better quality than the previous ones there, it's not awkwardly pinched together which is what you will get if you try to do something like a Network surface to an irregular boundary:



Then I used Edit > Trim to cut away these pieces:



Then use Edit > Join to glue those remaining pieces together and you should now be able to fillet like this:





Hope that helps, - Michael