Does anybody know the reason why fuselage and tail night textures don't light up?
Did the designer do night textures? Not all do - the plane just remains a 'dull' colour at night but you can still see the livery etc.
If so, is the tailfin supposed to light up? Not all do - in fact the majority of older aircraft don't have an illuminated tailfin.
If the answer to both of the above is yes - has the designer cocked up - or have file(s) been corrupted in the download?
The latter is not uncommon - I'm afraid that not all painters are fully experienced and quite a lot of stuff is uploaded with glitches and errors in it. In the early days (see my website) I had a Shorts 360 and when I asked why the night textures I had painted were faulty, Grumpy1 (Doug Attrell) was kind enough to point out to me that there was actually a model error, that the night textures themselves had not been properly applied and there was no way they would ever work without being fixed by the designer.
BTW - in any query like this you MUST tell us which aircraft is involved, which paint job and where you got it. It's pretty much useless just asking a 'general' question like you have - which is why my answer is so 'general'!

Roger