Hi Mark, probably related to these 2 lines here being apart by such a small distance kind of skimming by each other:



Zooming in there, those lines are apart from each other by a distance of 0.0006
units:



This kind of situation with pieces skimming by each other by such a small amount can make things a whole lot more difficult for the booleans to make sense of, it's a lot better for things to either be exactly aligned with one another or to be pushing through each other by a somewhat larger distance than that.

- Michael

Those lines also seem to be at a tiny angle to one another instead of going in the exact same direction.

Hi Mark, I've attached a file with those pieces fixed up.

I turned on control points for your curves and used the Transform > Align command to make those 2 lines shown above to be trued up with one another, and that helped the boolean to be able to figure it out better.

Things that are just barely misaligned by a small amount tend to make things difficult for the booleans - it has to try to decide whether to intersect such things with each other and leave little cut pieces behind, or whether to try and fuse some things together.

- Michael