Can't capture glide slope

[the_blackwell_ninja]

I tried using the APR autopilot mode for an ILS approach, and autopilot insisted on flying the plane as if trying to hold alt instead of descending on the glide-slope, though it would keep me on the right heading. I eventually just hand-flew it. As far as I know it's set up correctly. It's in NAV1 mode, localizer is live, and the glide slope and course are indicated on the instruments. The localizer has a glowing blue VLOC on it, and the indicator to the right of the autopilot modes has a glowing green "APP". What am I missing?

[Coop]

There are prerequisites to when the GS will actually arm (vs just getting LOC following with approach sensitivity). Make sure to only arm APR once the GS has started to be received.