Best Practices for Chamfers

Sometimes when I try to add a chamfer in Freecad 1.0.0, it simply doesn’t work on some edges. Can anyone give me a sense of what the basic criteria are in order for chamfers to work correctly and some of the common things that can go wrong? I generally don’t see any other problems in models that have edges that won’t chamfer and usually the same model has other edges that will chamfer. I know that making a chamfer too large will in some instances cause it to fail but I can’t quite put my finger on exactly why.