Have you put a straight edge on the board,(both bass and treble),and adjusted the truss rod so that the neck will flatten?This will show if or where the neck is warped.
Also put the edge across the sounboard in front,(neck side),of the bridge and below it.
If you sight down the neck it won't be very accurate for dimensions,but if you do that and look at the neck/body join you may spot a hump in the fretboard.
Quite often as the top shifts the fretboard may hump as well.Some places will raise the action to try to get around this,but if the to humps or dives then the problem will become obvious after tackling this.
Whatever the source of the problem is this isn't something you want to tackle as a first repair,(sorry),this kind of thing should be taken to a reputable shop to be evaluated.