UV Unwrapping makes a mess and misses faces I have a relatively simple mesh, that looks like this (a couple of tight faces and loop cuts due to subdivision smoothing):

However when I try to UV unwrap it, it's a whole mess and even misses most of the faces:

When I select a face I can see where it is located on the texture map:

But other faces aren't located on it at all:

What could have went wrong here?
I made sure to select all vertices of the mesh before unwrapping, so I don't think that's the problem.
You simply need to mark all the seems correctly and it will umwrap nicely. Make sure you mark all the small edges that could interfere with the unwrapping and try again.
Use ctrl+e while in edit mode to call the Edge menu where you can find Mark Seam.

It is also a good idea to apply scale of the object to get expected results when unwrapping. You can do that with ctrl+a with the object selected in object mode: