Hi yophie, your surfaces are just not close enough to each other to be joined there. Basically some surfaces are flat and some are curved, so they don't match up very well in their shapes and therefore won't join.

Here's a couple of screenshots to illustrate:



If you zoom in to these areas here you can see a fairly substantial gap between the shapes - that's because one shape is flat and one is curved:






In order for those to be joinable they have to be much closer to each other in shape where they meet up, usually that means either they should be created from a common construction curve (not different curves like one a polyline and one a bendy curve), or another possibility for a joinable juncture is to have 2 extended surfaces that are trimmed with each other.

I'll see if I can give you an example of how to construct a joinable result in a minute.

- Michael