Hi David, in order to join 2 surfaces they must have edges touching one another with no larger than a 0.005 unit gap between them. If there is too large of a space between them or if the edges are already joined to other things then either of those will prevent things from joining to one another.

If you have a case where join is not working as you had expected, please post the 3DM file with the things you are trying to join in it and then that would help to diagnose the specific problem that you are running into, it's much easier to tell you what is going wrong when looking at an actual model file rather than just a written description.

- Michael