0 AGL height means at the ground/sea level.
This causes the path calculation to generate an incorrect path. Therefore, the AI can't get into a vehicle that is on top of another object (e.g. a helicopter on top of a carrier or ship)
You can fix this by ensuring that the destination height is equal to the vehicle's AGL height (if the vehicle is touching the ground, of course, otherwise the path generation may fail for, e.g. a heli that is still landing). It's very easy to fix.