I am having trouble filleting this super easy and basic model. I really don´t understand why, I have done similar things a hundred times before and now it does not work. Can anyone tell me why?
This is the image… 
Have you double checked the top and bottom surfaces are part of the same polysurface that contains the edge you have filleted?
Any chance we can have a look at the original model if the problem persists?
Square box copy.3dm (131.0 KB)
It is so strange, today, making a new model, it worked and I dont´t see what I did different. I think the fillet function is very confusing. I send you the file “Square box copy”, the file from yesterday that did not work out, and the file from today that worked “Square box_1”.Square box_1.3dm (111.6 KB)
It looks to me like possibly you used FilletEdge in one case and FilletSrf in the other.
-Pascal
I think Pascal is on to it.
FilletSrf and FilletEdge are very different commands.
Here’s a link to a V4 White paper that will help you understand the differences so you aren’t confused:
http://wiki.mcneel.com/rhino/advancedfilleting
Some of the techniques shown are no longer needed in V5, but the techniques shown for how to “manually” get the results you want are worth understanding.