If you've ever considered installing low tension rounds on your bass it might be a good candidate to see if the string tension change improves your dead spots. It did on mine, but my original strings were extremely high tension. I replaced them with TI Jazz Rounds and immediately heard a difference.My fretless Yamaha BB-404F has a (noticeable to me) dead spot from 5th to 7th fret on the G string. The bass is 34", has a 2+2 tuner config, and is strung with regular tensioned D'Addario roundwounds (45...100).
Only conjecture on my part but as an engineer I'd assume the neck mass, neck flexibility and string mass all play a part in creating dead spots. Fixes like the Fat Finger and JohnK's (?) neat hidden weights both change the mass of the neck, stiffening bars may change the flexibility of the neck, light guage strings may change the string to neck mass ratio by a bit, and low tension strings may create less pull on the headstock in essence making the neck react like a stiffer neck. Again all guesses on my part.
So, if you're really annoyed by dead spots you might give some low tension strings a try, worked for me. YMMV