Hi Iwan, well there is probably no single cause that is happening to every one of these cases that you're describing.

It may be any variety of the reasons I mentioned above, or possibly others.

It is hard for me to give very specific advice without seeing any example models.

Filleting is generally a difficult area of calculation though, so it is not at all surprising to me that you will run into problems on more complex areas, especially when you have many edges coming together at one point.

If you need to do a lot of filleting of more complex models, you might want to look into one of the more expensive solid modelers like SolidWorks, Pro/E, etc... - they have invested a lot more time in their filleters so they are able to handle more complex cases than MoI.

Another thing you can do is to switch to surface modeling if you have a difficult fillet you need to solve. This involves using Edit / Separate to break your model up into individual surfaces, and then when you select 2 individual surfaces and do a fillet you will get a different style of fillet. This style does not try to hug directly along the edges and fix up corners where things meet up so it tends to be less prone to failing than the edge based fillets. You can often create a fillet surface with this method where the edge based one will fail to calculate. But then you do have to handle trimming things and trying to patch up corners manually.

The next time you run into a fillet issue if you post the model file here I can try to give you some better details.

Unfortunately filleting bugs are usually pretty difficult for me to solve myself, I rely on a geometry library to calculate these. I'm dependent on the people who make the library to fix filleting problems and make it more robust. They should be able to make progress in this area but it will be kind of a step by step thing.

- Michael