strange ILS indicator problem

here's the configuration:

SL30 talking to a TKM MC60 VOR/GS indicator.

On the SL30, the left-right indicator displays correctly when in cruise
and it detects a VOR.
On the MC60, the left-right displays correctly when in cruise and it
detects a VOR.

On the MC60, the glideslope indicator displays correctly when on approach.
But...on approach, there's no left-right indicator on the MC60.

Bench testing and in-aircraft testing by a Garmin shop shows no
problems with either radio or VOR/GS indicator. But I still don't
have the left-right on the MC60 when on approach.

Anyone got any suggestions?