There is nothing actually band sensitive in the KXAT1, other than there
is a unique L and C combination that would be used for any given antenna.
Might I suggest that you look carefully to see what might have changed
with your antenna. I believe that is the most probable answer.
You might also check the toroids in the KXAT1 - there could be one that
has a poorly tinned lead but worked OK for a while.
You could also verify that all relays operate by operating each relay
using the L0-L3 and C0-C3 along with NT1 and NT2 settings in the ATU
menu - check the relays for continuity across the closed contacts after
operating them. You will have to lift one leg of L1-L3 to check those
relays for continuity.
For antenna I use two wires, one vertical up to 25 ft, and one horizontal also up to 25 ft. The wires are adjusted if there is high SWR. I could get good SWR with this setup on 40m but not anymore. Nor problem with 30 or 20m.