Hi fraser, but also in order to be joined, the edges between 2 surfaces have to be touching pretty closely, they need to be within 0.005 units of one another.

There are some areas of your model that are not quite within that distance of one another.

For example this area here:





That gap is something like 0.0053 units in size, so it's just barely not close enough to be joined. I'll probably try to do something like recreate that vertical surface by lofting in a new one between the other ones edges. It looks like you may also be running into issues with the refitting mechanism not quite fitting tight enough to some of your shapes here, so setting the option in Loft for "Profiles: Exact" may help to avoid the fitting problem and get a more precise result. By default Loft can try to do a refitting process to try and produce more simplified surfaces but you can override that with the Profiles: Exact option.

- Michael