I'm not with Laminar Research, just a fellow simmer in the UK.
I haven't flown the Cessna 172 for a while as I usually fly the Zibo 737-800X or similar, but here a couple of thought starters:-
1. Have you set the active VLOC 1 frequency in the Garmin FMS (not sure which one you are using, possibly the 530?) to the ILS frequency for the airport approach in question?
2. Have you activated the approach in the Garmin FMS procedures page, before you get to the approach leg? I am not referring to the autopilot approach button here!
3. Are you entering the glide slope at the correct ILS platform altitude for the airport in question?
4. Have you set the heading bug on the heading indicator, to that of the runway you are attempting to land on?