Try giving the auto-tuner a little external assistance.

Connect a few micro-henrys or a capacitor in series or in parallel with the
transmission line at the antenna end of the tuner. This will often shift
the line input impedance into a range which the auto-tuner can accommodate.

Or try changing the length of the line by a few feet.


If the line input impedance is measured it can be deduced what sort of
additional component is needed for one awkward band. It may not adversely
affect other bands.


In the worst case situation you can always prune the antenna. Never give
up.
----
Reg, G4FGQ