Hi,
Easy way to figure if the issue is a deadspot is to gently push the headstock directly against a wall. This will change the resonance frequence of the neck, and the note will magically ring out, if it's a deadspot caused by resonance frequence.
I had a very disappointing PRS Custom 24 which had a total deadspot on G string octave, like zero sustain. On H-string the same note would transfer to a harmonic. Nothing could be done to that. Fat finger claims that their product could change the neck resonance, but it didn't work. Sold the guitar, buyer never complained, but I felt kind of bad. There are rumors that the neckheel of Custom 24 was changed to bigger version because of deadspot problems, but mine was a newer one with bigger heel.
I think every guitar has some weaker and sweeter sounding notes, but when it's really bad it becomes a deadspot. Usually the problem area is around G string octave, and most appearent on high gain.
My Suhr Modern has a weaker note on G# octave, but nowhere as bad as the Custom 24's G.