PMDG 747 Autoland help

ok, so I've been a veteran of the PMDG 747 (so i thought). I've been doing autolands for a while now and all have gone off without a hitch. but now i've got a problem. so i'm flying my usual route to lax from sfo, i select the ils for the 25's and i engage the localiser as normal and it holds. but when i get to engaging the approach, it shows up as approach, but the vnav remains on and will not dissengage. Therefore the autoland does not work. Now, I can do a manual at any time, but i'm stumped here. is it my altitude upon which i initially intercept (what is the optimum altitude anyways?), the speed, or the flight plan itself? something along those lines or whatever. if you might know, just reply i'm open to anything. thanks guys