Luke 1 neck problem

I have an older Luke 1, non compensated nut era that has a dip in the neck from about the 9th to 20th fret and the truss rod seems unable to straighten this area [ checked with slotted straight edge ]. The truss rod does have some range affecting the neck from the 1st to 9th , but not after that so can’t get a decent action because the frets after the 20th cause some buzzing.
Any good ideas on what is going on ? is it a truss rod problem, anyone ever encounter this? TIA