[Elecraft] question about using extenal ATU with KX3

[Julie Royster]

Smart folks, we have been using our KX3 fine with an LDG auto-tuner.  Today,
mysteriously, it fails to tune only the 20-meter band.  We checked the
antenna with SWR meter and it seems the same as previously.  The other bands
tune fine as before.

QUESTION:  is there some setting on the KX3 we could inadvertently have
changed that could cause this to occur?
THANKS for any input or ideas I can investigate!
Julie KT4JR