So I think you noticed that one surface cuts ok without any problem - the one that cuts more easily is this one here, note how it does not skim right along the knurl edges:




The one that has a lot more difficult looks like this - notice here the skimming type situation where it's trying to slice right through the same line as where the knurls have their own edges running in that same direction:




If you pull that problem one either up or down by just a little amount it will probably then intersect much more cleanly without a lot of edge coincidence happening to confuse things up.

- Michael