Hello, perhaps this could help you (Garmin manual) :
Prior to descent path interception, the Selected Altitude must be set below the current aircraft altitude by at least 75 feet. For the flight director to transition from Altitude Hold to Vertical Path Tracking Mode, acknowledgment is required within five minutes of descent path interception by:
• Pressing the VNV Key
• Adjusting the Selected Altitude
If you have set your new altitude more than 5 minutes before the TOD, nothing occurs. You have to turn the ALT button a bit to initiate your descent.