UV Unwrapping

Hi Guys.
I have a persistent problem with UV unwrapping. My workflow is to export as .fbx from Fusion 360, then apply materials etc in Blender. I have found that the smart UV unwrap works perfectly for me. But every once in a while the UV map for 1 of my solids seems to get stuck. Below are 2 objects that I have used exactly the same process on. Even deleting the UV map and starting again doesn’t seem to help.

The problem in mathematical sense is: there is no common solution because there are three dimensions mapped to two dimensions and nobody can tell for every posibility what to do.
So most of the time smart unwrap is really smart but the polgons in question are…
You tried to unwrap different groups of polygons to try different solutions for example deck, windows ,bridge …?

So as you say, each instance could be different.
In this case it seems I had double vertices that prevented the UV Unwrapping.

I have just made an account so I sadly can’t make my own post about my question, but it falls in the UV mapping/unwrapping category. So I’ll ask it here, hopefully this is allowed.
Also I’m very new to blender.

I want an image texture to overlap on specific faces of different objects. Is there a easy way to fix this?
I’m creating a stage with different screens in the background, (the weird blue & black spikes), and I want the video I put as a material to show the entire video on all of the spikes, instead of every spike showing the same part of the video. I know I can use UV mapping to do this manually, but is there a way to easily do this? Since it’s so confusing, difficult and tedious to do it per face of every object…