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