Hi Ella, the booleans seem to be having trouble with the "seam edge" of the vertical cylinder just grazing along the other surface:





I'll try to investigate it a bit, but it may be something that will require tuning from the people who make the geometry library that I am using. That kind of grazing situation can often be a difficult area in intersection calculations.

Currently to make it work the easiest thing is to rotate the vertical cylinder by 90 degrees so that seam edge is "out of the mix" for the intersection, see attached file boolean-bug2.3dm which should now boolean ok.

- Michael