I am trying to fillet the edges of closed solid polysurface (I’m making a pair of glasses and trying to fillet the edges of the arms). I’m using the FilletEdge command and the edges do fillet however one face disappears. Why is this and how can I resolve it. Thanks for any help 
ps I’m a total beginner with rhino
Post a .3dm file with the object you are trying to fillet. You can create the file using the Export command. To upload the file click on the vertical arrow icon above where you type a post.