Here is the UVs in Blender.
Heres the UVs in Substance Painter. Also, why isnt the entrance showing up? Is it because it is a extruded egde?
The most obvious answer is that there’s actually another hidden object in your mesh that shows up as the huge hexagon overlaying almost all other islands in Substance Painter. Try going into edit mode on your object and hitting Alt-H to unhide everything, then check if that n-gon shows up in the UV editor. As for the “entrance”, is it a single polygon with no thickness? Then in Substance only one side of it will be visible. Try orbiting around in Substance to see if the thing can be seen from another angle.
I think your problem could be normal. CTRL+N is the reverse/ flip all normals to one direction. Select the faces that are not appearing and try CTRL+N. Or turn on the mattecap shaders and see if it is a different color than the rest.
Du er fra norge?