Hi Gian,
Here is another possible explanation (I don't know about the 747 but had this with other aircraft).
The AP+FD depends on a command that is present in some panels (but not all) as "Avionics Main Switch". The catch is that if this "switch" is off, the AP+FD won't engage, and of course there is no obvious way to turn it on if no physical button exists.
The workaround is to get into Settings / Commands in FS and map a key combination to this command, then within the cockpit use this key combination to enable avionics before engaging AP+FD.
I don't think it's related to ProPilot, the easiest way to get into this trouble is flying an aircraft WITH avionics main button, switching it off then switching to an aircraft WITHOUT this command. And don't forget FS weird behaviour concerning what it remembers as defaults at startup.
Hope this helps.
Andrei