Hi Tony - just loading your file and filleting the bottom piece works fine for me over here:



Are you possibly trying to use too large of a radius value?

For a fillet on this piece you've got some quite narrow areas on it and that will limit the fillet to a radius of not larger than around 0.02 units, any larger than that and the pieces from different sides will start to collide into one another:



Fillet will have problems if you try to ask for a fillet radius that is larger than will actually fit in the available space in the model.

Sometimes it can be surprising just how much room a fillet takes up - if fillet does not work with your first radius value it is often a good idea to try some smaller and smaller values, like 1/10 of your initial value, then 1/20, 1/50, etc... just to see if that was the problem.

- Michael