What has worked for me is the A/P must be in NAV flying the flight plan, the approach is activated on the GTN and the aircraft is on the approach, the GTN must transition to LPV usually at the FAF, the glide slope cue appears on the pdf, and I select APP on the mode control panel as the glide slope cue begins to sink. It sounds like a lot of steps but they all flow sequentially so long as the A/P is in NAV and flying the approach. You should also be sure to look at the approach plate to be sure you cross the IAF and FAF at the correct altitudes.